Fix is_fsdp_enabled() logic and make low_cpu_mem_usage by default
Author: helloworld1
Pull request: yes

# What does this PR do?
is_fsdp_enabled()  logic is incorrectly using "FSDP_CPU_RAM_EFFICIENT_LOADING" envvar, which is only set by accelerate launcher not in the TrainingArguments. This PR removes the condition and only look for "ACCELERATE_USE_FSDP" to determine if fsdp enabled or not.

The "FSDP_CPU_RAM_EFFICIENT_LOADING" now only deciding if low_cpu_mem_usage is used or not.

This fixed the issue that is_fsdp_enabled() is returning False if using `torchrun` to launch the script making low_cpu_mem_usage unusable.


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Comments:
> Hello @helloworld1, Thank you for the proposed change. However, `low_cpu_mem_usage` can't be default as there were models such as Whisper and few others that were resulting in errors with this being default.

Got it. Thanks for pointing out! @pacman100 Do you think it is a good idea to make `low_cpu_mem_usage` configurable through `fsdp_config`?

Currently environment variable `FSDP_CPU_RAM_EFFICIENT_LOADING` is being read but not set in `transformers` library. I can add a `low_cpu_mem_usage` config to set the environment variable so it will apply the low mem.

I proposed this PR: https://github.com/huggingface/transformers/pull/30002 to add the FSDP config. 

@pacman100 @muellerzr I am closing this PR in favor of this PR https://github.com/huggingface/transformers/pull/30002 to make low cpu mem usage option configurable. Thanks!