Initial XLA support for TF eager. This is prerequisite for TF compiler's XLA support.

Initial XLA support for TF eager. This is prerequisite for TF compiler's XLA support.

This CL adds XLA support for the following TFE_Op's:

1. A TF op such as MatMul, with full support of constant and resource params.

2. A TF_Function as TFE_Op, where the function must have no constant and
resource params. Removing this restriction requires more discussion and will be
deferred to a later time.

PiperOrigin-RevId: 184877345