de-duplication strategy in non-eager mode.

TF, under certain circumstances, prepends `_#` numbers to variables with the
same name. This appears to happen in non-eager mode and in some occasions with
Keras. TFF previously used these names for the type signature, which would
cause problems when the same model was created more than once in the same graph
context.

Instead, rely on the ordering of the sequence of variables contained in the
model and use the unnamed structure for types.

PiperOrigin-RevId: 285032395