eager: Add tests for backprop.py

This includes some changes to operator overloading:
(a) TensorNode is more consistent with the operator overloads in Tensor
(b) Additionally, it has __eq__ and __ne__ overloads which were needed
    to make the transition from ops.embedding_lookup to tf.nn.embedding_lookup
    possible. However, this is inconsistent with overloads for Tensor
    so is something that will be looked at in a follow up.

PiperOrigin-RevId: 165816199