Add support for 2D PatchEmbeddingBlock, ViT and UNETR

[ahatamiz]

Is your feature request related to a problem? Please describe.

• Extend current implementations of PatchEmbeddingBlock, ViT and UNETR for 2D applications (e.g. classification, segmentation)

Describe the solution you'd like

• Extend current functionality in PatchEmbeddingBlock for tokenizing 2D inputs.

• Add workflow tutorials for how to use 2D PatchEmbeddingBlock with ViT model.

• Extend UNETR and UNETR_blocks to support 2D inputs (i.e. segmentation)

part of #2394, follow-up of #2476