experiment(backend): autocast dtype in CustomLinear

This resolves an issue where specifying `float32` precision causes FLUX Fill to error.

I noticed that our other customized torch modules do some dtype casting themselves, so maybe this is a fine place to do this? Maybe this could break things...

See #7836

Author: psychedelicious

invokeai/backend/model_manager/load/model_cache/torch_module_autocast/cast_to_dtype.py

@@ -0,0 +1,19 @@
+from typing import TypeVar
+
+import torch
+
+T = TypeVar("T", torch.Tensor, None, torch.Tensor | None)
+
+
+def cast_to_dtype(t: T, to_dtype: torch.dtype) -> T:
+    """Helper function to cast an optional tensor to a target dtype."""
+
+    if t is None:
+        # If the tensor is None, return it as is.
+        return t
+
+    if t.dtype != to_dtype:
+        # The tensor is on the wrong device and we don't care about the dtype - or the dtype is already correct.
+        return t.to(dtype=to_dtype)
+
+    return t

invokeai/backend/model_manager/load/model_cache/torch_module_autocast/custom_modules/custom_linear.py

@@ -3,6 +3,7 @@
 import torch
 
 from invokeai.backend.model_manager.load.model_cache.torch_module_autocast.cast_to_device import cast_to_device
+from invokeai.backend.model_manager.load.model_cache.torch_module_autocast.cast_to_dtype import cast_to_dtype
 from invokeai.backend.model_manager.load.model_cache.torch_module_autocast.custom_modules.custom_module_mixin import (
     CustomModuleMixin,
 )
@@ -73,6 +74,10 @@ def _autocast_forward_with_patches(self, input: torch.Tensor) -> torch.Tensor:
     def _autocast_forward(self, input: torch.Tensor) -> torch.Tensor:
         weight = cast_to_device(self.weight, input.device)
         bias = cast_to_device(self.bias, input.device)
+
+        weight = cast_to_dtype(weight, input.dtype)
+        bias = cast_to_dtype(bias, input.dtype)
+
         return torch.nn.functional.linear(input, weight, bias)
 
     def forward(self, input: torch.Tensor) -> torch.Tensor: