Add planning stage in mkl_fft

[sanjiyan3]

Any plan to add a planning stage ? That is very useful when mkl_fft is used inside a loop to reduce runtime.

[oleksandr-pavlyk]

Yes, actually. In MKL's DFTI interface such plans are called descriptors. mkl_fft already caches descriptors for 1D transforms, but it is a good change to expose the committed descriptor as a Python object and allow to pass it to respective Python functions.