Improvements to eager linear regression benchmark:

  1. Using _shape_tuple
  2. Bypassing * over math_ops.mul etc
  3. Flatmaps in the tape code
  4. Cache for ones similar to for zeros
  5. Fast path for _SubGrad
  6. Fast global_step += 1 for resource variables
  7. Bypassing deprecated args decorator in eager mode

PiperOrigin-RevId: 183446593