Simplify input transform object

[ethanwharris]

What does this PR do?

Makes the following simplifications:

• Removes the input_ and target_ prefixed versions of the hooks. This massively simplifies the logic around resolving transforms and creates a clear method heirarchy where train, val, test etc. prefixes by default just call to the non-prefixed version. As a result, we can remove all of the is_overridden checks and other resolutions.
• Removes support for passing a callable. This was not used in our codebase and is not recommended as it's not clear how the callable will be handled.

Before submitting

• Was this discussed/approved via a Github issue? (no need for typos and docs improvements)
  Discuss it here 😃 In this case it was easier to show than tell
• 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.
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Make sure you had fun coding 🙃

[codecov]

Codecov Report

Merging #1340 (c6fdf33) into master (ee512a2) will increase coverage by 0.10%.
The diff coverage is 97.56%.

@@            Coverage Diff             @@
##           master    #1340      +/-   ##
==========================================
+ Coverage   92.67%   92.78%   +0.10%     
==========================================
  Files         286      285       -1     
  Lines       12814    12671     -143     
==========================================
- Hits        11876    11757     -119     
+ Misses        938      914      -24     

Flag	Coverage Δ
unittests	92.78% <97.56%> (+0.10%)	⬆️

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

Impacted Files	Coverage Δ
flash/core/data/io/input_transform.py	93.59% <95.74%> (+5.08%)	⬆️
flash/audio/classification/input_transform.py	92.30% <100.00%> (ø)
flash/image/classification/input_transform.py	83.87% <100.00%> (-0.51%)	⬇️
flash/image/face_detection/input_transform.py	100.00% <100.00%> (ø)
flash/image/segmentation/input_transform.py	93.75% <100.00%> (-6.25%)	⬇️
flash/image/style_transfer/input_transform.py	100.00% <100.00%> (ø)
flash/template/classification/data.py	95.77% <100.00%> (-0.12%)	⬇️
flash/text/question_answering/model.py	93.19% <0.00%> (-0.69%)	⬇️
flash/core/serve/dag/task.py	97.88% <0.00%> (+1.05%)	⬆️
... and 1 more

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