Make Filter a Collection instead of a Container

[NeilGirdhar]

This is a minor change to repair an error in the definition of FIlter: Container is not Sized, but needs to be for len to work. It should probably have been a typing.Collection.

Checklist

• This PR fixes a minor issue (e.g.: typo or small bug) or improves the docs (you can dismiss the other
  checks if that's the case).
• This change is discussed in a Github issue/
  discussion (please add a
  link).
• The documentation and docstrings adhere to the
  documentation guidelines.
• This change includes necessary high-coverage tests.
  (No quality testing = no merge!)

[codecov-commenter]

Codecov Report

Merging #1815 (a21204f) into main (addfe9c) will not change coverage.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##             main    #1815   +/-   ##
=======================================
  Coverage   83.17%   83.17%           
=======================================
  Files          69       69           
  Lines        5974     5974           
=======================================
  Hits         4969     4969           
  Misses       1005     1005           

Impacted Files	Coverage Δ
flax/core/scope.py	89.09% <100.00%> (ø)

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[jheek]

Looks good!