Add tutorial to show different sampling transforms

[Nic-Ma]

Is your feature request related to a problem? Please describe.
Currently, we already have several crop transforms that can generate random samples/patches during training, I think we should develop a tutorial to show how to use them and the expected data that generated.
Like: RandCropByPosNegLaheld, RandSpatialCropSamplesd, RandWeightedCropd and the RandCropByLabelClassesd(in PR: Project-MONAI/MONAI#2557).

[Nic-Ma]

If @wyli @ericspod @rijobro don't have other comments, @yiheng-wang-nv could you please help develop this tutorial when we finalized all the tasks for v0.6?

Thanks in advance.

[Nic-Ma]

Hi @yiheng-wang-nv ,

As the RandCropByLabelClasses transform had been merged, I think you can start this ticket now.

Thanks in advance.