[mlgo][nfc] Decouple TensorSpec from tensorflow.

The motivation is twofold:

1) Allow plugging in a different training-time evaluator, e.g.
   TFLite-based, etc.

2) Allow using TensorSpec for AOT, too, to support evolution: we start
   by extracting a superset of the features currently supported by a
   model. For the tensors the model does not support, we just return a
   valid, but useless, buffer. This makes using a 'smaller' model (less
   supported tensors) transparent to the compiler. The key is to
   dimension the buffer appropriately, and we already have TensorSpec
   modeling that info.

The only coupling was due to the reliance of a TF internal API for
getting the element size, but for the types we are interested in,
`sizeof` is sufficient.

A subsequent change will yank out TensorSpec in its own module.

Differential Revision: https://reviews.llvm.org/D124045