Add "sub_module" argument in PretrainedTransformerMismatchedEmbedder

[sythello]

Fixes this question: https://stackoverflow.com/questions/71167641/in-allennlp-how-can-we-set-sub-module-argument-in-pretrainedtransformermismat

Changes proposed in this pull request:

• Added "sub_module" argument in PretrainedTransformerMismatchedEmbedder initializer.

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[dirkgr]

Thanks for doing this! Can you add a Changelog entry?

[sythello]

Hi, thank you for the quick response! I have added an entry in CHANGELOG.md, please check.

[dirkgr]

Fantastic!