[Feature] More callbacks

[matteobettini]

Introduced a training callback (that can be used for example to compute custom losses) and a callback for rendering during evaluation

[codecov-commenter]

Codecov Report

Attention: 10 lines in your changes are missing coverage. Please review.

Comparison is base (ea835d0) 89.17% compared to head (21cd5fd) 83.29%.

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #35      +/-   ##
==========================================
- Coverage   89.17%   83.29%   -5.88%     
==========================================
  Files          53       53              
  Lines        2420     2437      +17     
==========================================
- Hits         2158     2030     -128     
- Misses        262      407     +145     

Files	Coverage Δ
benchmarl/experiment/logger.py	89.05% <ø> (-2.92%)	⬇️
benchmarl/experiment/experiment.py	89.44% <83.33%> (-2.49%)	⬇️
benchmarl/environments/common.py	66.66% <66.66%> (ø)
benchmarl/experiment/callback.py	62.16% <50.00%> (-7.07%)	⬇️

... and 13 files with indirect coverage changes

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