Fix checkpoint converter missing parallel group initialization

[yashaswikarnati]

Add fake process groups required by ProcessGroupCollection during checkpoint conversion. Also add accelerate to mlm optional deps.

Test:

uv pip install -e ".[mlm]"
python tools/checkpoint/convert.py \
  --bf16 --model-type GPT --loader llama_mistral --saver core \
  --target-tensor-parallel-size 1 --checkpoint-type hf \
  --load-dir <hf-model-path> --save-dir ./converted \
  --tokenizer-model <hf-model-path> --model-size mistral

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[faradawn]

Hi @yashaswikarnati , it successful resolved the issue of data parallel group uninitialized. Thanks a lot!

[faradawn]

Tested successfully for Mixtral model

[faradawn]

This PR adds more initialization: https://github.com/NVIDIA/Megatron-LM/pull/3111/changes. Potentially safer?

[dimapihtar]

LGTM. Thank you!