make clamp range of PowerMeanAggregation adjustable

[KaneGreen]

1. Add two args when class init to allow users to set their own clamp function range.
2. Change the default lower bound of clamp function from 0 to 0.0001. This is because the value of 0 can easily cause p to become NaN after several forwards when learn=True.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 85.71%. Comparing base (c211214) to head (a4a8f40).
Report is 71 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master   #10366      +/-   ##
==========================================
- Coverage   86.11%   85.71%   -0.40%     
==========================================
  Files         496      501       +5     
  Lines       33655    34456     +801     
==========================================
+ Hits        28981    29534     +553     
- Misses       4674     4922     +248     

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[KaneGreen]

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