Update howto_use_tfp.md

[junpenglao]

Simplify model building in TFP

Thank you for opening a PR!

A few important guidelines and requirements before we can merge your PR:

• If I add a new sampler, there is an issue discussing it already;
• We should be able to understand what the PR does from its title only;
• There is a high-level description of the changes;
• There are links to all the relevant issues, discussions and PRs;
• The branch is rebased on the latest main commit;
• Commit messages follow these guidelines;
• The code respects the current naming conventions;
• Docstrings follow the numpy style guide
• pre-commit is installed and configured on your machine, and you ran it before opening the PR;
• There are tests covering the changes;
• The doc is up-to-date;
• If I add a new sampler* I added/updated related examples

Consider opening a Draft PR if your work is still in progress but you would like some feedback from other contributors.

[codecov]

Codecov Report

Merging #553 (d3113a3) into main (fd2cae8) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##             main     #553   +/-   ##
=======================================
  Coverage   99.14%   99.14%           
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  Files          49       49           
  Lines        2099     2099           
=======================================
  Hits         2081     2081           
  Misses         18       18