Better description of the metric arg

[SkafteNicki]

What does this PR do?

Fixes #361
Adds better description of the metric arg for all tasks. Additionally, to me it seems that for the following task the metric arg has no effect (because they do not use the step method from the base Task class)

• Object detection
• seq2seq summarization
• seq2seq translation

have added this to the docs but need confirmation from someone else that this is actually true.

Before submitting

• Was this discussed/approved via a Github issue? (no need for typos and docs improvements)
• Did you read the contributor guideline, Pull Request section?
• Did you make sure your PR does only one thing, instead of bundling different changes together?
• Did you make sure to update the documentation with your changes?
• Did you write any new necessary tests? [not needed for typos/docs]
• Did you verify new and existing tests pass locally with your changes?
• If you made a notable change (that affects users), did you update the CHANGELOG?

PR review

• Is this pull request ready for review? (if not, please submit in draft mode)

Anyone in the community is free to review the PR once the tests have passed.
If we didn't discuss your PR in Github issues there's a high chance it will not be merged.

Did you have fun?

Make sure you had fun coding 🙃

[codecov]

Codecov Report

Merging #473 (a0032c3) into master (06ee84e) will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master     #473   +/-   ##
=======================================
  Coverage   91.85%   91.85%           
=======================================
  Files         112      112           
  Lines        7068     7069    +1     
=======================================
+ Hits         6492     6493    +1     
  Misses        576      576           

Flag	Coverage Δ
unittests	91.85% <100.00%> (+<0.01%)	⬆️

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
flash/image/detection/model.py	83.11% <ø> (ø)
flash/image/classification/model.py	86.66% <100.00%> (ø)
flash/image/embedding/model.py	82.45% <100.00%> (ø)
flash/image/segmentation/model.py	87.87% <100.00%> (ø)
flash/tabular/classification/model.py	93.02% <100.00%> (ø)
flash/text/classification/model.py	93.18% <100.00%> (+0.15%)	⬆️
flash/text/seq2seq/core/model.py	73.91% <100.00%> (ø)
flash/text/seq2seq/summarization/model.py	78.94% <100.00%> (ø)
flash/text/seq2seq/translation/model.py	75.00% <100.00%> (ø)
flash/video/classification/model.py	86.25% <100.00%> (ø)

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update 06ee84e...a0032c3. Read the comment docs.

[ethanwharris]

This is great, thanks @SkafteNicki 😃 what do you suggest about the unused args? I think for now it would be good to remove them, do you want to do that in another PR?

[SkafteNicki]

@ethanwharris it depends, if we think that in the future these tasks should have a working metrics argument then it is maybe better to just leave them as they are, instead of going through the process of deprecating them :]
If not, then lets just remove them.

[ethanwharris]

@SkafteNicki Yeah, that makes sense, I've opened an issue (#480) to get them working with metrics.