Convert to markdown: named_tensor.rst, nested.rst, nn.attention.bias.rst, nn.attention.experimental.rst, nn.attention.flex_attention.rst

@sekyondaMeta

Convert the following files to MyST markdown and ensure that the doc tests are green:

To convert an .rst file:

Assign the issue to yourself using the /assigntome comment.
Important! Use git mv filename.rst filename.md to convert the issue from .rst to .md. DO NOT delete
the .rst and create the .md - we won't accept PRs with deleted .rst files. We want to preserve the git log of
those files. Therefore, using git mv is very important!
Before submitting a PR, build the docs locally as described in the README.
Preview localy with make serve to ensure that the HTML renders correctly.

Run lintrunner from the root of the repository:

pip install lintrunner
lintrunner init
lintrunner -m main

Submit a PR with your changes and ensure the PR passes the doc CI tests. Specifically it needs to pass:
- pull / linux-docs / build-docs-python-false (pull_request)
- linux-jammy-py3.9-gcc11 / test (docs_test, 1, 1, lf.linux.2xlarge)
Doc tests start after the Docker build, which might take about 30 minutes.
The doc build itself should take about 25 minutes. When the build is done,
you can see the preview of your build by clicking:
📄 Preview Python docs built from this PR in the pytorch-bot comment.

Make sure your PR is tagged with docathon-h1-2025, module: docs, topic: not user facing,
and topic: docs labels.
Compare the preview of the pages you converted with same pages it the main branch.
The converted pages in preview must look the same as in the main branch.
Once the PR is accepted, one of the maintainers will merge it.

Here is an example PR

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