add parameter prepare_together_function to WrappedMetricWithPrepareFunction

[ArneBinder]

... and make prepare_function optional. From the docstring:

prepare_function: A function that prepares the input for the metric. If provided, It is called with
    the predictions as well as the targets (separately).
prepare_together_function: A function that prepares both the predictions and the targets together and
    should return them as a tuple. If provided, it is called with the predictions and the targets as
    arguments.

Motivation: This can be used to remove true negatives (TNs) from metric calculation.

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 92.00000% with 2 lines in your changes missing coverage. Please review.

Project coverage is 95.63%. Comparing base (3c31d85) to head (475fd0d).

Files with missing lines	Patch %	Lines
...es/metrics/wrapped_metric_with_prepare_function.py	92.00%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #129      +/-   ##
==========================================
- Coverage   95.68%   95.63%   -0.05%     
==========================================
  Files          61       61              
  Lines        5077     5089      +12     
==========================================
+ Hits         4858     4867       +9     
- Misses        219      222       +3     

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