Adding a LanguageModelHead abstraction (#3200)

* Modules and docs

* Added tests

* Docstrings

* pylint

* moved linear layer to tests

* add todos about caching

* fix import...

* doc

Author: matt-gardner

allennlp/modules/__init__.py

@@ -24,3 +24,4 @@
 from allennlp.modules.input_variational_dropout import InputVariationalDropout
 from allennlp.modules.bimpm_matching import BiMpmMatching
 from allennlp.modules.residual_with_layer_dropout import ResidualWithLayerDropout
+from allennlp.modules.language_model_heads import LanguageModelHead

allennlp/modules/language_model_heads/__init__.py

@@ -0,0 +1,3 @@
+from allennlp.modules.language_model_heads.language_model_head import LanguageModelHead
+from allennlp.modules.language_model_heads.bert import BertLanguageModelHead
+from allennlp.modules.language_model_heads.gpt2 import Gpt2LanguageModelHead

allennlp/modules/language_model_heads/bert.py

@@ -0,0 +1,37 @@
+from overrides import overrides
+from pytorch_transformers import BertConfig, BertForMaskedLM
+import torch
+
+from allennlp.modules.language_model_heads.language_model_head import LanguageModelHead
+
+
+@LanguageModelHead.register('bert')
+class BertLanguageModelHead(LanguageModelHead):
+    """
+    Loads just the LM head from ``pytorch_transformers.BertForMaskedLM``.  It was easiest to load
+    the entire model before only pulling out the head, so this is a bit slower than it could be,
+    but for practical use in a model, the few seconds of extra loading time is probably not a big
+    deal.
+    """
+    def __init__(self, model_name: str) -> None:
+        super().__init__()
+        config = BertConfig.from_pretrained(model_name)
+        self.input_dim = config.hidden_size
+        self.output_dim = config.vocab_size
+        # TODO(mattg): It's possible that we could use some kind of cache like we have in
+        # allennlp.modules.token_embedders.bert_token_embedder.PretrainedBertModel.  That way, we
+        # would only load the BERT weights once.  Though, it's not clear how to do that here, as we
+        # need to load `BertForMaskedLM`, not just `BertModel`...
+        bert_model = BertForMaskedLM.from_pretrained(model_name)
+        self.bert_lm_head = bert_model.cls  # pylint: disable=no-member
+
+    @overrides
+    def get_input_dim(self) -> int:
+        return self.input_dim
+
+    @overrides
+    def get_output_dim(self) -> int:
+        return self.output_dim
+
+    def forward(self, hidden_states: torch.Tensor) -> torch.Tensor:
+        return self.bert_lm_head(hidden_states)

allennlp/modules/language_model_heads/gpt2.py

@@ -0,0 +1,37 @@
+from overrides import overrides
+from pytorch_transformers import GPT2Config, GPT2LMHeadModel
+import torch
+
+from allennlp.modules.language_model_heads.language_model_head import LanguageModelHead
+
+
+@LanguageModelHead.register('gpt2')
+class Gpt2LanguageModelHead(LanguageModelHead):
+    """
+    Loads just the LM head from ``pytorch_transformers.GPT2LMHeadModel``.  It was easiest to load
+    the entire model before only pulling out the head, so this is a bit slower than it could be,
+    but for practical use in a model, the few seconds of extra loading time is probably not a big
+    deal.
+    """
+    def __init__(self, model_name: str) -> None:
+        super().__init__()
+        config = GPT2Config.from_pretrained(model_name)
+        self.input_dim = config.hidden_size
+        self.output_dim = config.vocab_size
+        # TODO(mattg): It's possible that we could use some kind of cache like we have in
+        # allennlp.modules.token_embedders.bert_token_embedder.PretrainedBertModel.  That way, we
+        # would only load the GPT2 weights once.  Though, it's not clear how to do that here, as we
+        # need to load `GPT2LMHeadModel`, not just `GPT2Model`...
+        gpt2_model = GPT2LMHeadModel.from_pretrained(model_name)
+        self.gpt2_lm_head = gpt2_model.lm_head  # pylint: disable=no-member
+
+    @overrides
+    def get_input_dim(self) -> int:
+        return self.input_dim
+
+    @overrides
+    def get_output_dim(self) -> int:
+        return self.output_dim
+
+    def forward(self, hidden_states: torch.Tensor) -> torch.Tensor:
+        return self.gpt2_lm_head(hidden_states)

allennlp/modules/language_model_heads/language_model_head.py

@@ -0,0 +1,19 @@
+import torch
+
+from allennlp.common import Registrable
+
+
+class LanguageModelHead(torch.nn.Module, Registrable):
+    """
+    A ``LanguageModelHead`` encapsulates a function that goes from some hidden state to logits over
+    a vocabulary.
+    """
+    def get_input_dim(self) -> int:
+        raise NotImplementedError
+
+    def get_output_dim(self) -> int:
+        raise NotImplementedError
+
+    def forward(self, hidden_states: torch.Tensor) -> torch.Tensor:  # type: ignore
+        # pylint: disable=arguments-differ
+        raise NotImplementedError

allennlp/tests/modules/language_model_heads/__init__.py

@@ -0,0 +1,19 @@
+# pylint: disable=invalid-name,no-self-use,protected-access
+import torch
+
+from allennlp.common import Params
+from allennlp.common.testing.test_case import AllenNlpTestCase
+from allennlp.modules.language_model_heads import LanguageModelHead, BertLanguageModelHead
+
+
+class TestBertLanguageModelHead(AllenNlpTestCase):
+    def test_can_init_and_run(self):
+        # The LM head code reads a module from somewhere else; we're basically just testing here
+        # that we can initialize the expected model `from_params`.
+        head = LanguageModelHead.from_params(Params({"type": "bert", "model_name": "bert-base-uncased"}))
+        assert isinstance(head, BertLanguageModelHead)
+        assert head.get_input_dim() == 768
+        assert head.get_output_dim() == 30522
+        tensor = torch.rand(1, 768)
+        logits = head(tensor)
+        assert tuple(logits.size()) == (1, 30522)

allennlp/tests/modules/language_model_heads/bert_test.py

@@ -0,0 +1,19 @@
+# pylint: disable=invalid-name,no-self-use,protected-access
+import torch
+
+from allennlp.common import Params
+from allennlp.common.testing.test_case import AllenNlpTestCase
+from allennlp.modules.language_model_heads import LanguageModelHead, Gpt2LanguageModelHead
+
+
+class TestGpt2LanguageModelHead(AllenNlpTestCase):
+    def test_can_init_and_run(self):
+        # The LM head code reads a module from somewhere else; we're basically just testing here
+        # that we can initialize the expected model `from_params`.
+        head = LanguageModelHead.from_params(Params({"type": "gpt2", "model_name": "gpt2"}))
+        assert isinstance(head, Gpt2LanguageModelHead)
+        assert head.get_input_dim() == 768
+        assert head.get_output_dim() == 50257
+        tensor = torch.rand(1, 768)
+        logits = head(tensor)
+        assert tuple(logits.size()) == (1, 50257)

allennlp/tests/modules/language_model_heads/gpt2_test.py

@@ -0,0 +1,35 @@
+from overrides import overrides
+import torch
+
+from allennlp.data import Vocabulary
+from allennlp.modules.language_model_heads.language_model_head import LanguageModelHead
+
+
+@LanguageModelHead.register('linear')
+class LinearLanguageModelHead(LanguageModelHead):
+    """
+    Uses ``torch.nn.Linear`` as a language model head.  Does nothing else fancy.  This was intended
+    largely for testing code with small models and simple components.  It's likely that you would
+    want something nicer for actually training a language model, such as tying weights with an
+    input embedding, or an adaptive softmax, or something.  But, if you find this class useful for
+    something you're doing and want it moved into the repo, open an issue on github.
+    """
+    def __init__(self,
+                 vocab: Vocabulary,
+                 input_dim: int,
+                 vocab_namespace: str) -> None:
+        super().__init__()
+        self.input_dim = input_dim
+        self.output_dim = vocab.get_vocab_size(vocab_namespace)
+        self.linear = torch.nn.Linear(self.input_dim, self.output_dim)
+
+    @overrides
+    def get_input_dim(self) -> int:
+        return self.input_dim
+
+    @overrides
+    def get_output_dim(self) -> int:
+        return self.output_dim
+
+    def forward(self, hidden_states: torch.Tensor) -> torch.Tensor:
+        return self.linear(hidden_states)

allennlp/tests/modules/language_model_heads/linear.py

@@ -0,0 +1,22 @@
+allennlp.modules.language_model_heads
+=====================================
+
+.. automodule:: allennlp.modules.language_model_heads
+   :members:
+   :undoc-members:
+   :show-inheritance:
+
+.. automodule:: allennlp.modules.language_model_heads.language_model_head
+   :members:
+   :undoc-members:
+   :show-inheritance:
+
+.. automodule:: allennlp.modules.language_model_heads.bert
+   :members:
+   :undoc-members:
+   :show-inheritance:
+
+.. automodule:: allennlp.modules.language_model_heads.gpt2
+   :members:
+   :undoc-members:
+   :show-inheritance:

doc/api/allennlp.modules.language_model_heads.rst

@@ -14,6 +14,7 @@ allennlp.modules
    allennlp.modules.lstm_cell_with_projection
    allennlp.modules.elmo
    allennlp.modules.elmo_lstm
+   allennlp.modules.language_model_heads
    allennlp.modules.conditional_random_field
    allennlp.modules.feedforward
    allennlp.modules.highway