[tf.data] Adding optimization for fusing consecutive `map(...)` transformations.

This optimization pass fuses maps together e.g:

  map(x: x*x).map(x: x+1)

can be fused to:
  map(x: x*x+1)

We can see 10-x speedup on simple pipeline containing 50 consecutive maps of identity.

PiperOrigin-RevId: 206069355