Avoid retaining two copies of each constant in `ConstantOp`.

Avoid retaining two copies of each constant in `ConstantOp`.

Presently, the kernel keeps two copies of the constant tensor value, which can be large:
1. In the `ConstantOp::tensor_` field.
2. In the `OpKernel::def_` field (as an attr of the `NodeDef`).

Since we can be sure that `ConstantOp` will never need to access the
tensor value from `OpKernel::def_`, this change introduces a mechanism
for `OpKernel` implementations to store a stripped `NodeDef` in the
base class, and uses it in `ConstantOp` to avoid storing the tensor
value attr.

PiperOrigin-RevId: 184455793