Validation progress bar with metrics

[ethanjperez]

🚀 Feature

Logging validation metrics throughout the duration of validation (e.g., current batch loss/acc or average batch loss/acc so far).

Motivation

If the validation set is large, it'd helpp to know right away if I've loaded the wrong checkpoint or loaded a checkpoint in the wrong way within a couple iterations, rather than waiting to run through the entire validation set.

Pitch

I want to be able to see basically the same progress bar during training as I do during validation/testing.

Alternatives

Not sure of any alternatives - this is quite helpful for debugging and is probably a not-too-large change since training already supports this feature.

[github-actions]

Hi! thanks for your contribution!, great first issue!

[williamFalcon]

in summary, we need to support:

validation_step(...):
  return {'log':{...}}

[Borda]

@ethanjperez mind submit a PR?

[williamFalcon]

@PyTorchLightning/core-contributors anyone?

[Borda]

will it it be also solved in #1989?