flash attention import error is getting masked

[RobotSail]

We still use the flash_attn_varlen_func even when we failed to import it so we get the following error:

[rank3]:     attn_output = flash_attn_varlen_func(
[rank3]:                   ^^^^^^^^^^^^^^^^^^^^^^
[rank3]: NameError: name 'flash_attn_varlen_func' is not defined

But this error usually arises as a fault of not being able to install flash attention for any reason. So when we cannot import flash attention, we should either avoid using this function or we should throw an error indicating why we can't import flash attention.