Add a single positional argument mode for shape inference in subclassed Models.

Allows fit() when call's signature looks something like call(x, training=True).

Calling conventions are "inputs", single positional, and multiple positional. Right now the distinction between "inputs" and single positional calling conventions is the text of one error message. Both support shape inference (which just hasn't been implemented for multiple positional input arguments yet).

PiperOrigin-RevId: 198815483