Fix: Ensure environment variable values are case-insensitive in Accelerate

[jp1924]

What does this PR do?

# YAML configuration that can be passed to TrlParser's parse_args_and_config
env:
  ACCELERATE_USE_FSDP: true
  FSDP_VERSION: 2
  FSDP_RESHARD_AFTER_FORWARD: true
  FSDP_STATE_DICT_TYPE: "FULL_STATE_DICT"
  FSDP_AUTO_WRAP_POLICY: "TRANSFORMER_BASED_WRAP"
  FSDP_CPU_RAM_EFFICIENT_LOADING: false
  FSDP_ACTIVATION_CHECKPOINTING: false

When passing ACCELERATE_USE_FSDP: true in the YAML config file and running the script, the os.environ.get("ACCELERATE_USE_FSDP", "false") == "true" condition in Accelerate does not activate FSDP due to receiving the value "True" (with uppercase "T").

To prevent this issue, all string values obtained via os.environ.get should be converted to lowercase using .lower().

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Who can review?

@SunMarc

[SunMarc]

Thanks a lot !

[HuggingFaceDocBuilderDev]

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[jp1924]

thank you!