remove unnecessary call to `F.pad` (#10620)
* rewrite memory count without implicitly using dimensions by @ic-synth
* replace F.pad by built-in padding in Conv3D
* in-place sums to reduce memory allocations
* fixed trailing whitespace
* file reformatted
* in-place sums
* simpler in-place expressions
* removed in-place sum, may affect backward propagation logic
* removed in-place sum, may affect backward propagation logic
* removed in-place sum, may affect backward propagation logic
* reverted change
