[FSDP] Make env var and dataclass flag consistent for cpu_ram_efficient_loading

[SumanthRH]

What does this PR do?

Makes environment variable and dataclass consistent for cpu_ram_efficient_loading. Currently, there's an inconsistency when the FSDP plugin is initialized in a standalone fashion: the cpu_ram_efficient_loading flag can be set to True but if the user doesn't use accelerate launch , then the environment variable FSDP_CPU_RAM_EFFICIENT_LOADING would not be set. This environment variable is actually used by transformers to initialize parameters efficiently in the process group (See: #2100 (comment) for details on how this happens)

I can see a previous PR #2998 that tried to make all env vars optional, but looks like this was left.

Fixes # (issue)

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cc @muellerzr