new python file to upload mosaic-bert to hf

[Patchwork53]

• Added convert_mosaicbert_to_hf.py inside inference folder.

• inference/convert_composer_to_hf.py was only able to convert MPT and other CausalLMs to HF. Adapted that code to work on mosaic_bert

• The python script takes in an optional bert_config_path JSON, if not provided, it uses the config from mosaicml/mosaic-bert-base-Huggingface. It also downloads (via the requests package) some required scripts such as bert_layers.py from there.

• Wasn't sure whether to PR on this repository or mosaicml/examples. Decided to do it here since my script needs the llmfoundry package anyway.

• Solves MosaicBERT: Convert composer weights to HF examples#445 and Accessing model after pre-training examples#401 (comment)