* Created opset7.md to add the specification of the operation FFT.
* Started to write the specification of the operation FFT.
* Added link to the specification of the operation FFT.
* Continued to write the specification of the FFT.
* Written about inputs and outputs of FFT.
* Started to write examples.
* Added example when there is no input 'signal_size'.
* Added more example.
* Small fixes.
* Small fix.
* Renamed FFT to DFFT.
* Small fix.
* Small fix.
* Started to write the algorithm of FFT.
* Added asserts.
* Started to write the method __call__ of the class of DFFT calculation.
* Fixed category.
* Continued to write the algorithm of the FFT calculation.
* Continued to write the algorithm of the FFT calculation.
* Continued to write the algorithm of the FFT calculation.
* Continued to write the algorithm of the calculation of DFFT.
* Written the algorithm of the calculation of DFFT.
* Small fix.
* Renamed operation.
* Added examples of 3D input tensors.
* Covered complex number representation.
* Written formulas for FFT.
* Written about start point of trimming.
* Small fixes.
* Small fixes.
* Some fixes.
* Added some note.
* Added a description of the calculation of the output shape.
* Added examples with unsorted axes and with (-1) in signal_size.
* Fixed range of axes indices.
* Small fix.
* Small change.
* Added T_SIZE type.
* Added negative axes support.
* Some fixes.
* Some fixes.
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