Add gradient for SVD. This is based on draft code by Catalin Ionescu...

Add gradient for SVD. This is based on draft code by Catalin Ionescu (cdi@google.com), using the algorithm outlined in Mike Giles' paper: http://eprints.maths.ox.ac.uk/1079/1/NA-08-01.pdf.

This initial version has the following restrictions:
  Only supports statically known inner matrix dimensions m and n.

Backpropagating through U and V (i.e. backpropagating through SVD nodes with compute_uv=True) has further restrictions:
  a) Only supports real tensors.
  b) Only supports square and "almost square" matrices where the number of rows and columns differ by at most 1.
  c) full_matrices must be true also. This does not currently have severe implications, given the restriction in b).

Feature request on Github:
#6503

This CL also adds support for calling tf.real, tf.imag, and tf.angle with real arguments.

PiperOrigin-RevId: 171836140