[BUG] default snapshot backend doesn't match docs

[jannessm]

Describe the bug

The documentation doesn't match its implementation.

When using the trainer as described, the trainer cannot save a checkpoint since the provided path is a directory and not a file.

To Reproduce

Try to use the trainer with save_file_file = './ckpts'. Of course the directory must exist.

Expected behavior

Saved checkpoints by torchsnapshot.

[vmoens]

I will correct this thanks for flagging.