Add min_bler and callback to sim_ber

[nbecker]

Description

This adds 2 features to sim_ber function.

1. Add min_bler stopping feature. Similar to early_stop, if the BLER on the previous ebno step was less than some value then stop. This is useful if, for example, you only want to simulate down to 1e-5.
2. Add callback. This will call a callback function at each iteration, which can be used for logging, for example.

• Fixes a bug?

Describe what was wrong, and explain the solution in detail.

• Adds a new feature?

Yes

Describe the new feature, ensure that it is properly documented. Note that for new features unit tests are required.

• Introduces API changes?
  No

Address in detail why it is needed. Explain and describe the consequences, does it break older code? Note that API changes might be delayed until a major bump release for inclusion.

This is a non-breaking addition which will help with long-running simulations.

• Other contributions

Please detail the nature of the submission.

Checklist

[x ] Detailed description
[ ] Added references to issues and discussions
[ ] Added / modified documentation as needed
[ ] Added / modified unit tests as needed
[ ] Passes all tests
[ ] Lint the code
[ ] Performed a self review
[ x] Ensure you Signed-off the commits. Required to accept contributions!
[ ] Co-authored with someone? Add Co-authored-by: user@domain and ensure they signed off their commits too.

[SebastianCa]

Thank you for the PR. We will review the code in the next days and provide feedback.

[SebastianCa]

We have reviewed the PR and added some functions for early stopping via callbacks.
Please note that we have removed the min_bler parameter as this has been already internally integrated in the next release of Sionna.

The PR is ready to be merged.

[SebastianCa]

And of course thank again for your contribution to Sionna.

[nbecker]

Note that although callback might be used for early stopping that was not my purpose for it, I added it to do logging to see incremental results while sim is running, to be able to produce plots from running sims.

[SebastianCa]

That's correct, but there is no harm in having the additional possibility of advanced early stopping. One example could be stopping the simulation after a certain time or when the error variance is below a certain threshold.