Add Swin UNETR model

[ahatamiz]

Is your feature request related to a problem? Please describe.
Swin UNETR [1] has achieved SOTA performance in various datasets such as MSD and BTCV. It would be nice to have it implemented in MONAI.

[1]: Tang, Yucheng, Dong Yang, Wenqi Li, Holger Roth, Bennett Landman, Daguang Xu, Vishwesh Nath, and Ali Hatamizadeh. "Self-Supervised Pre-Training of Swin Transformers for 3D Medical Image Analysis." arXiv preprint arXiv:2111.14791 (2021).

Describe the solution you'd like
A PR that brings in all the components into MONAI..