Adding a new config parameter to combine layers during FSDP

[tejasnagendra]

Created a new class called MultiBlock, which wraps around multiple Block to reduce the number of NCCL communication. Number of blocks to combine can be controlled with num_blocks_to_combine parameter GPT/Llama.

Ideally the message size should be around 1GB to get the best performance. But when models are running on multiple nodes every layer is split into really small chunks causing the message size to be extremely small resulting in suboptimal usage of network bandwidth. This parameter can be controlled to make sure we send larger message sizes.

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