Update Polynomial Tensor example with torch api (#3884)

Fixes# #3879

## Description
torch.pi available since PyTorch 1.8.0 so use it instead of extra math
call.

The changes made are tested and made sure that it produce the desired
output.

Signed-off-by: Sahdev Zala <spzala@us.ibm.com>
Co-authored-by: sekyondaMeta <127536312+sekyondaMeta@users.noreply.github.com>
Co-authored-by: Svetlana Karslioglu <svekars@meta.com>

Author: 3 people

beginner_source/examples_tensor/polynomial_tensor.py

@@ -19,15 +19,13 @@
 """
 
 import torch
-import math
-
 
 dtype = torch.float
 device = torch.device("cpu")
 # device = torch.device("cuda:0") # Uncomment this to run on GPU
 
 # Create random input and output data
-x = torch.linspace(-math.pi, math.pi, 2000, device=device, dtype=dtype)
+x = torch.linspace(-torch.pi, torch.pi, 2000, device=device, dtype=dtype)
 y = torch.sin(x)
 
 # Randomly initialize weights