Move tasks code inside entry files

[ashleve]

What does this PR do?

Moves tasks code inside entry files. Removes the tasks folder.
Related to #404

Motivation:

• Each entry file is always paired with task file so the split is redundant.
• More minimalistic template

Drawbacks:

• Slower shell tab completion due to no longer existing possibility of nesting imports inside @hydra.main decorated function
• pyrootutils.setup_root() now gets executed every time the task method is imported in other files (which can happen in tests) because the line must be placed at the beginning of the file. This shouldn't break the tests though as far as I'm aware.

Before submitting

• Did you make sure title is self-explanatory and the description concisely explains the PR?
• Did you make sure your PR does only one thing, instead of bundling different changes together?
• Did you list all the breaking changes introduced by this pull request?
• Did you test your PR locally with pytest command?
• Did you run pre-commit hooks with pre-commit run -a command?

Did you have fun?

Always 🙃

[codecov-commenter]

Codecov Report

Merging #421 (37e86ce) into main (c4dde48) will increase coverage by 1.68%.
The diff coverage is 94.44%.

@@            Coverage Diff             @@
##             main     #421      +/-   ##
==========================================
+ Coverage   82.90%   84.59%   +1.68%     
==========================================
  Files          11        9       -2     
  Lines         351      344       -7     
==========================================
  Hits          291      291              
+ Misses         60       53       -7     

Impacted Files	Coverage Δ
src/eval.py	90.32% <92.85%> (+90.32%)	⬆️
src/train.py	95.83% <95.45%> (-4.17%)	⬇️

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