Bump Megatron LM example from 0.4 to 0.7 (or latest as option)

[perifaws]

The MegatronLM sample is set to 0.4 and needs an update to a later version starting at 0.7. The version should be configurable with a default in the docker file (top).

This is the part to update but there will be dependencies changes, for example on transformers and NCCL + OFI.

RUN cd /workspace && git clone --depth 1 --branch core_v0.4.0 https://github.com/NVIDIA/Megatron-LM.git \
	&& cd Megatron-LM \
	&& python3 -m pip install nltk  \
	&& python -m pip install .