Small changes in `perplexity.rst`to make the notebook executable on google collaboratory (#13541)

* add imports

* Update docs/source/perplexity.rst
Co-authored-by: Sylvain Gugger <35901082+sgugger@users.noreply.github.com>
Co-authored-by: Sylvain Gugger <35901082+sgugger@users.noreply.github.com>

docs/source/perplexity.rst

@@ -110,6 +110,9 @@ available to condition on).
 
 .. code-block:: python
 
+    import torch
+    from tqdm import tqdm
+
     max_length = model.config.n_positions
     stride = 512