Allow MegatronPretrainingRandomSampler to do multi-epoch training
jenkins jenkins
# What does this PR do ?

Allows the MegatronPretrainingRandomSampler class to correctly handle multi-epoch training by calculating length correctly when consumed_samples > total_samples

**Collection**: NLP

# Changelog 
- nemo/collections/nlp/data/language_modeling/megatron/data_samplers.py: modified MegatronPretrainingRandomSampler to correctly return length for multi-epoch training when consumed_samples > total_samples
- nemo/collections/nlp/data/language_modeling/megatron/megatron_batch_samplers.py: fixed len calculation to be correct when drop_last=False, and also cleaned up constructor logic caused by these fixes: https://github.com/NVIDIA/NeMo/pull/7728/files#diff-26575f60355f81991aef1f635e947a368b086afe220c5a81817e35b1c00b2e38

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