Set default axes for N-dimensional fft APIs

[yasahi-hpc]

In this PR, we set default axes for ND fft APIs.
For example, for 2D Views, default axes = {-2, -1}.

using axes_type = KokkosFFT::axis_type<2>;
axes_type axes  = {-2, -1};
KokkosFFT::fftn(execution_space(), x, out);
KokkosFFT::fftn(execution_space(), x, out, axes);

[cedricchevalier19]

Can you be more explicit in the PR title? ND is not very clear. "N-D" or "N-dimensional" is easier to understand.

[yasahi-hpc]

Can you be more explicit in the PR title? ND is not very clear. "N-D" or "N-dimensional" is easier to understand.

Thank you for your review. I have fixed accordingly. May I have your second review?

[tpadioleau]

Seems good to me, thanks for this feature

[yasahi-hpc]

Sure, I will merge this. Thank you for your help