Set depth_multiplier default value, so that one can use separable_conv2d and...

Set depth_multiplier default value, so that one can use separable_conv2d and conv2d interchangeably (conv2d does not have depth_multiplier arg).
For example:
  if use_separable:
    conv_op = separable_conv2d
  else:
    conv_op = conv2d
  net = conv_op(....)
Most common usage of separable conv use depth_multiplier=1, e.g. in MobileNet and Xception.

PiperOrigin-RevId: 207741004