Expose merge_param at top level of flax.linen.

[n2cholas]

Fixes #1319 to let users access merge_param through flax.linen.merge_param.

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!)

[jheek]

Hi, the build failed due to a dependency resolution problem with the latest tensorflow release. Could you rebase this PR please?

[n2cholas]

@jheek I rebased but the run still failed. For some reason I can't see the details of why it failed, all I can see is an "X" with "This check failed".

[codecov-commenter]

Codecov Report

Merging #1324 (bf1c2df) into master (c53c1c5) will not change coverage.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master    #1324   +/-   ##
=======================================
  Coverage   82.33%   82.33%           
=======================================
  Files          65       65           
  Lines        5316     5316           
=======================================
  Hits         4377     4377           
  Misses        939      939           

Impacted Files	Coverage Δ
flax/linen/__init__.py	100.00% <100.00%> (ø)

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[marcvanzee]

@jheek I rebased but the run still failed. For some reason I can't see the details of why it failed, all I can see is an "X" with "This check failed".

I think it actually succeeded, but it still shows the previous run that failed.