Add instructions for GPU-enabled PyTorch to installation.md (#5600)

Fixes #5333 .

### Types of changes
<!--- Put an `x` in all the boxes that apply, and remove the not
applicable items -->
- [x] Documentation updated, tested `make html` command in the `docs/`
folder.

Signed-off-by: Dženan Zukić <dzenan.zukic@kitware.com>
Co-authored-by: Mingxin Zheng <mingxinz@nvidia.com>

Author: dzenanz

docs/source/installation.md

@@ -26,6 +26,11 @@ To install optional features such as handling the NIfTI files using
 Ignite](https://pytorch.org/ignite/), please follow the instructions:
 - [Installing the recommended dependencies](#installing-the-recommended-dependencies)
 
+The installation commands bellow usually end up installing CPU variant of PyTorch. To install GPU-enabled PyTorch:
+1. Install the latest NVIDIA driver.
+1. Check [PyTorch Official Guide](https://pytorch.org/get-started/locally/) for the recommended CUDA versions. For Pip package, the user needs to download the CUDA manually, install it on the system, and ensure CUDA_PATH is set properly.
+1. Continue to follow the guide and install PyTorch.
+1. Install MONAI using one the ways described below.
 ---