Fix overwriting FP8 act ckpt flag in the train script

[cli99]

The current activation checkpointing with FP8 TE requires using the non-reentrant implementation from TE. The fsdp_config for activation checkpointing with TE FP8 is

  fsdp_config:
    activation_checkpointing: true
    activation_checkpointing_reentrant: false
    activation_cpu_offload: false
    te_checkpoint_wrapper: true

The activation_checkpointing_reentrant param in yaml get overwritten in the train.py script due to legacy reasons. Need to rewrite the check.

[mvpatel2000]

Thanks! Sorry for the bad merge