Updated trainer lifecycle

[orperel]

Contains updates to the trainer lifecycle and better docs for tracers:

• After changing trainer to "steps by iterations" rather than by epochs, the gui step counter was bugged - this is fixed by this MR.
• Added pre_train / post_train functions which are useful for logging (i.e. shutting off the wandb connection)
• Some logging logic (mostly wandb) shifted to the trainer
• A fix to df.parquet which couldn't dump torch tensors
• Some extra args in trainer are handled more carefully to avoid crashes for the upcoming reformatted config parser

Signed-off-by: operel operel@nvidia.com