AutoencoderKL docstrings have reference to num_channels instead of channels

**Describe the bug**
In AutoencoderKL (https://github.com/Project-MONAI/MONAI/blob/dev/monai/networks/nets/autoencoderkl.py) the docstrings of the Encoder, Decoder and AutoencoderKL refer to num_channels although the argument is called channels. 
