update user guide to include admonition on mypy config

docs/source/user_guide.md

@@ -23,6 +23,20 @@ Additional dependency groups can be installed using the `--with` flag. For examp
 python3 -m poetry install --with docs,test
 ```
 
+```{admonition} mypy configuration options
+:class: important
+
+By default, the `mypy` configuration in the `pyproject.toml` disallows subclassing
+the `Any` type - `allow_subclassing_any = false`. In cases where the type checker
+is not able to determine the types of objects in some external library (e.g. `PyTorch`),
+it will treat them as `Any` and raise errors. If your codebase has many of such
+cases, you can set `allow_subclassing_any = true` in the `mypy` configuration or
+remove it entirely to use the default value (which is `true`). For example, in 
+a `PyTorch` project, subclassing `nn.Module` will raise errors if `allow_subclassing_any`
+is set to `false`.
+```
+
+
 ## pre-commit
 
 You can use [pre-commit](https://pre-commit.com/) to run pre-commit hooks (code checks,