make make_rng default to 'params'

[chiamp]

Following #3669.

The Flax team has decided to switch the default make_rng name to 'params' instead of 'default'.

This would have the following effect (see the original proposal with 'default' in #3668):

• Make self.make_rng(name='params') callable without explicitly passing in a string name, in which it would default to the 'params' RNG stream
  • This would allow the user to pass in a single RNG mapping (e.g. {'params': random.key(0)}) to .init() and .apply() that self.make_rng('other_rng_stream') will use as the RNG seed if, for example, {'other_rng_stream': random.key(1)} wasn't in the RNG mapping that was passed into .init() or .apply()

[codecov-commenter]

Codecov Report

Attention: 2 lines in your changes are missing coverage. Please review.

Comparison is base (736d133) 59.10% compared to head (c6b5de0) 59.10%.

Files	Patch %	Lines
flax/core/scope.py	33.33%	2 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #3699   +/-   ##
=======================================
  Coverage   59.10%   59.10%           
=======================================
  Files         103      103           
  Lines       12426    12426           
=======================================
  Hits         7345     7345           
  Misses       5081     5081           

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