automodsumm don't work with templates?

I get the following error when I try to use` automodsumm ` with a template when building matplotlib.
`jinja2.exceptions.TemplateAssertionError: no filter named 'underline'`. The error happen in the call
```
template = template_env.get_template(template_name)
```
on line 511.

I looked into the code and saw that the line
```
template_env.filters['underline'] = _underline
```
where in the sphinx `generate_autosummary_docs` function but not in the `generate_automodsumm_docs` function.

I added that line and the` _underline` function and I don't get the error anymore. The result is more as expected but sphinx doesn't render the last level of `autosummary` files (if that makes any sense, the links on my class pages don't work).

Start by comparing generate_automodsumm_docs with Sphinx's generate_autosummary_docs, focusing on template_env.get_template at line 511 and the reported underline filter registration. Reproduce the matplotlib template build, then verify both the template error and the missing links on the last autosummary level are resolved.

Written by the indexing model from the issue text.