Removed redundant str() call in _maybe_define_function.

It was causing a significant slowdown when either of the input tensors
had <1000 elements (which is a NumPy heuristic for collapsing the str
with ...)

Reproducer:

def square(X):
  return tf.matmul(X, X, transpose_a=True)

square_f = tf.function(square)

>>> X = tf.zeros([42, 24])  # 1008 elements.
>>> _ = square_f(X)
>>> %timeit square_f(X)
10000 loops, best of 3: 167 ?s per loop

>>> X = tf.zeros([42, 1])
>>> _ = square_f(X)
>>> %timeit square_f(X)
100 loops, best of 3: 1.54 ms per loop

PiperOrigin-RevId: 237459548