Suggesting updates on the documentation of `DenseNet`

[ParsifalXu]

Hi,

It seems that there is a potential incomplete description of the document of DenseNet(Similar issues also exist in Xception and ResNet). As mentioned in the document:

classes: optional number of classes to classify images into, only to be specified if include_top is True, and if no weights argument is specified.

However, corresponding part in the code:

if weights == "imagenet" and include_top and classes != 1000:
    raise ValueError(
        'If using `weights` as `"imagenet"` with `include_top`'
        " as true, `classes` should be 1000"
    )

Although this will not cause a bug, because the default value is 1000, I think this default value should be added like similar documents(MobileNet) to reduce unnecessary understanding.

[joehiggi1758]

@ParsifalXu I'll give this one a shot and circle back shortly!

[joehiggi1758]

@ParsifalXu I've updated the requested from above and this has been merged into master!