Move FFT ops under the spectral namespace.

[nsthorat]

tf.fft is now tf.spectral.fft, to match TensorFlow: https://www.tensorflow.org/api_docs/python/tf/spectral/fft

We do not support tf.fft as an alias, as this is the old way to do it in TensorFlow (the other spectral ops do not have an alias in the top-level namespace).

Note that the chain API does not change, users still call tensor.fft().

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This change is 

[caisq]

Reviewable status: 0 of 1 approvals obtained (waiting on @caisq and @dsmilkov)

[dsmilkov]

Reviewed 4 of 4 files at r1.
Reviewable status: complete! 1 of 1 approvals obtained (waiting on @nsthorat and @dsmilkov)

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src/ops/spectral_ops.ts, line 32 at r1 (raw file):

 * const x = tf.complex(real, imag);
 *
 * x.fft().print();

add in a comment // tf.spectral.fft(x).print(); (like we do for other ops)

[nsthorat]

Reviewable status: complete! 2 of 1 approvals obtained

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src/ops/spectral_ops.ts, line 32 at r1 (raw file):

Previously, dsmilkov (Daniel Smilkov) wrote…

add in a comment // tf.spectral.fft(x).print(); (like we do for other ops)

Done