Add torch.cos() term entry for PyTorch with syntax and example

content/pytorch/concepts/tensor-operations/terms/cos/cos.md

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+# torch.cos()
+
+## Introduction
+`torch.cos()` is a PyTorch tensor operation that computes the cosine of each element in the input tensor.
+
+It is part of PyTorch’s extensive mathematical operations, which enable element-wise computations on tensors commonly used in scientific computing and deep learning.
+
+## Syntax
+```python
+torch.cos(input, *, out=None) → Tensor
+
+## Example
+
+import torch
+
+# Create a tensor with values in radians
+x = torch.tensor([0, torch.pi / 2, torch.pi])
+
+# Compute the cosine
+y = torch.cos(x)
+
+print(y)
+# tensor([ 1.0000,  0.0000, -1.0000])