Need Optional parameter for Keypoint class as we have one for BoundingBox

In case, we want to track the Keypoint transformations and pickout the labels which we want to use, It would be useful if we add label parameter.

As of now, we can do something like the below code in BB's :
`ia.BoundingBox(
x

So, we have some existing code over at the PyTorch Ignite project that is actually pretty general and might be really handy to have in DALI: pytorch/ignite#766

In core PyTorch, you can chain transformations and FileIO together easily with the Compose() operation: https://pytorch.org/docs/stable/torchvision/transforms.html#torchvision.transforms.Compose

Something like

The last thing to do before registering v0.6.0 in the General registry is to update the documentation.

@Evizero Looks like this repository haven't set up auto-deployment CI for documentation, right? Can you add a deploy key and a secret key (with name DOCUMENTER_KEY) following the instruction of https://juliadocs.github.io/Documenter.jl/stable/man/hosting/#travis-ssh-1 I plan to set the docum