[update] Install PyTorch on Ubuntu 20.04

[Rajakavitha1]

Fixes: #6265
Tested and validated the commands and updated the guide.

rajie@pytorch:~/anaconda$ python3
Python 3.8.10 (default, May 26 2023, 14:05:08) 
[GCC 9.4.0] on linux
Type "help", "copyright", "credits" or "license" for more information.
>>> import torch
>>> print (torch.cuda.is_available)
<function is_available at 0x7fcb6a241040>
>>> print (torch.cuda.is_available())
False
>>> print (torch.cuda.device_count)
<function device_count at 0x7fcb6a25f940>
>>> print (torch.cuda.device_count())
0
>>> 

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