update transposition to latest torch convention

[matthieutrs]

Updating a transposition to latest torch convention that was raising a deprecation warning message at each call of the transform and clogging training logs.

The warning was:

The use of `x.T` on tensors of dimension other than 2 to reverse their shape is deprecated and it will 
throw an error in a future release. Consider `x.mT` to transpose batches of matrices or 
`x.permute(*torch.arange(x.ndim - 1, -1, -1))` to reverse the dimensions of a tensor.

This PR implements precisely the second suggestion. I couldn't find any other use of .T in the repo but I'd happily update if you point me to others :)

[bruAristimunha]

Nice!

Thank you very much for your PR! Could you register your modification in the what's news file for you to be listed as contributors?

https://raw.githubusercontent.com/braindecode/braindecode/master/docs/whats_new.rst

[codecov]

Codecov Report

Merging #446 (6b33ad9) into master (c2a8312) will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master     #446   +/-   ##
=======================================
  Coverage   84.11%   84.11%           
=======================================
  Files          59       59           
  Lines        4268     4269    +1     
=======================================
+ Hits         3590     3591    +1     
  Misses        678      678           

[matthieutrs]

Sure @bruAristimunha! I've updated

[bruAristimunha]

Thank you @matthieutrs