[TF] Extend the Softmax kernels so they accept shapes with rank >= 1, rather...

[TF] Extend the Softmax kernels so they accept shapes with rank >= 1, rather than relying on the caller to reshape to rank 2.

Guard the Python library code that reshapes softmax inputs to rank 1 with a forward compatibility check; after the forward compatibility window expires, the Python code will no longer reshape to rank 1.

PiperOrigin-RevId: 207606326