Added builder functions for zeros and ones initializers

[chiamp]

Addresses part of #2002 and is mentioned by @marcvanzee here.

Added builder functions for zeros and ones initializers.

[marcvanzee]

Nice! I think it makes sense to, as a follow-up, replace all occurrences of "zeros" and "ones" in our codebase for initializing parameters with the new initializer functions, WDYT?

[codecov-commenter]

Codecov Report

Merging #2790 (871638e) into main (52be1fd) will increase coverage by 0.04%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##             main    #2790      +/-   ##
==========================================
+ Coverage   81.18%   81.23%   +0.04%     
==========================================
  Files          53       53              
  Lines        5645     5659      +14     
==========================================
+ Hits         4583     4597      +14     
  Misses       1062     1062              

Impacted Files	Coverage Δ
flax/linen/initializers.py	100.00% <100.00%> (ø)
flax/linen/__init__.py	100.00% <0.00%> (ø)
flax/linen/transforms.py	94.14% <0.00%> (+0.21%)	⬆️

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