DOC Output shape documentation

[MuawizChaudhary]

I add some words stating that the output shape is dependent on the max order. I also fix the given calculation of output shape for 3d.

Fixes Issue #454 and #639

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 88.03%. Comparing base (ca9073f) to head (83116f8).
Report is 40 commits behind head on dev.

Additional details and impacted files

@@           Coverage Diff           @@
##              dev     #820   +/-   ##
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  Coverage   88.03%   88.03%           
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  Files          64       64           
  Lines        2323     2323           
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  Hits         2045     2045           
  Misses        278      278           

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[lostanlen]

how can a doc change cause FAILED tests/scattering2d/test_tensorflow_backend_2d.py::TestFFT::test_fft[TensorFlowBackend2D] ?

[MuawizChaudhary]

how can a doc change cause FAILED tests/scattering2d/test_tensorflow_backend_2d.py::TestFFT::test_fft[TensorFlowBackend2D] ?

@lostanlen this test has been failing for python 3.7, pytorch 1.8, but its a tensorflow test??? so I dont get it. @janden got any hints?

Edit: seems related to #663

[MuawizChaudhary]

@janden @edouardoyallon have bandwidth to review?