[feat] Add BasePredictionWriter 3/3

[tchaton]

What does this PR do?

Here is PR 1/2: #7141
Here is PR 2/3: #7215

This PR adds BasePredictionWriter.

        import torch
        import os
        from pytorch_lightning.callbacks import BasePredictionWriter

        class CustomWriter(BasePredictionWriter):

            def __init__(self, output_dir: str, write_interval: str):
                super().__init__(write_interval)
                self.output_dir

            def write_on_batch(
                self, trainer, pl_module: 'LightningModule', prediction: Any, batch_indices: List[int], batch: Any,
                batch_idx: int, dataloader_idx: int
            ):
                torch.save(prediction, os.path.join(self.output_dir, dataloader_idx, f"{batch_idx}.pt")

            def write_on_epoch(self, trainer, pl_module: 'LightningModule', predictions: List[Any], batch_indices: List[Any]):
                torch.save(predictions, os.path.join(self.output_dir, "predictions.pt")

Fixes #7113

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[pep8speaks]

Hello @tchaton! Thanks for updating this PR.

There are currently no PEP 8 issues detected in this Pull Request. Cheers! 🍻

Comment last updated at 2021-04-27 19:59:37 UTC

[codecov]

Codecov Report

Merging #7127 (b3f60b8) into master (c6d9f52) will decrease coverage by 4%.
The diff coverage is 98%.

@@           Coverage Diff           @@
##           master   #7127    +/-   ##
=======================================
- Coverage      91%     87%    -4%     
=======================================
  Files         198     199     +1     
  Lines       12728   12772    +44     
=======================================
- Hits        11614   11145   -469     
- Misses       1114    1627   +513     

[ethanwharris]

LGTM 😃 small changes

[ananthsub]

lgtm!

[Borda]

does not have any effect here in py files
cc: @awaelchli

[awaelchli]

@tchaton the pr shows many coverage warnings

[awaelchli]

need to be careful that we don't overload these test helper functions with complexity. If the test functions are too complex we would need tests for the tests and so this goes in circles xD

[Borda]

also rather keep them protected so noone would import them...