No-ZeRO reshaping

[Muennighoff]

Should be merged first: https://github.com/bigscience-workshop/Megatron-DeepSpeed/pull/239
Only adds tools/convert_checkpoint/deepspeed_to_deepspeed_nozero.py

Our small models are trained without ZeRO. This script enables reshaping of them.

Tests:

• Loss continues where it left off after reshaping from
  • PP=4, TP=4 -> PP=2, TP=2 👍
  • PP=4, TP=4 -> PP=1, TP=1 👍
  • PP=2, TP=1 -> PP=1, TP=1 👍
• Checkpoint size stays the same 👍

Notes:

• I'm not doing any black formatting etc, as this is not a production codebase - Let me know if that's not okay & the code should be cleaner!