enh: change output_volume based on post_merge_function

[stelladk]

• Change the output_volume based on potential resizing in the post_merge_function
• Allow arbitrary reshaping of the output without affecting the post-activity (reshape_function attribute). This change handles flattening when growing Conv -> Lin.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.

Flag	Coverage Δ
unittests	94.55% <100.00%> (-0.72%)	⬇️

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Files with missing lines	Coverage Δ
src/gromo/modules/conv2d_growing_module.py	90.59% <100.00%> (+0.16%)	⬆️

... and 1 file with indirect coverage changes

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[TheoRudkiewicz]

Sorry, but I don't understand this PR. So I can't review it.

[sylvchev]

Thanks @stelladk