Add an `implementation` argument to `tf.keras.layers.LocallyConnected2D` and...

Add an `implementation` argument to `tf.keras.layers.LocallyConnected2D` and `tf.keras.layers.LocallyConnected1D`. The new mode (`implementation=2`) performs forward pass as a single dense matrix multiplication, allowing dramatic speedups in certain scenarios (but worse performance in others - see docstring). The option also allows to use `padding=same`.

PiperOrigin-RevId: 208737074