Return also classes for MAP metric

[SkafteNicki]

What does this PR do?

Fixes #1417
Returns also classes for MAP metric.

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[codecov]

Codecov Report

Merging #1419 (7cf0962) into master (4f3cab9) will decrease coverage by 54%.
The diff coverage is 100%.

Additional details and impacted files

@@           Coverage Diff            @@
##           master   #1419     +/-   ##
========================================
- Coverage      90%     36%    -54%     
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  Files         211     211             
  Lines       10849   10850      +1     
========================================
- Hits         9719    3870   -5849     
- Misses       1130    6980   +5850     

[adamjstewart]

Unfortunately this makes it much more difficult to use this metric in Lightning.

The following code:

class Foo(LightningModule):
    def __init__(self):
        self.metric = MeanAveragePrecision()

    def on_validation_epoch_end(self):
        self.log_dict(self.metric.compute())

used to work in 0.11.4. However, this PR adds a new classes key-pair to the metrics dict containing a list of all observed classes. log_dict now complains about this entry:

ValueError: `self.log(classes, tensor([0, 1], dtype=torch.int32))` was called, but the tensor must have a single element. You can try doing `self.log(classes, tensor([0, 1], dtype=torch.int32).mean())`

Here, tensor([0, 1]) is the classes value. Current workaround is to delete this key from the dict before logging, but it would be good to do this automatically.