Update warmup log messages and comments

[tjohnson31415]

Description

This came out of a follow up to #270 to determine why an extra Prefill was necessary after using warmup_mode. I learned that the extra Prefill is required to deploy the compiled graph to the Spyre device. This PR does not change any functionality, but updates logging and documentation around warmup to make this clearer.

[github-actions]

👋 Hi! Thank you for contributing to vLLM support on Spyre.
Just a reminder: Make sure that your code passes all the linting checks, otherwise your PR won't be able to be merged. To do so, first install the linting requirements, then run format.sh and commit the changes. This can be done with uv directly:

uv sync --frozen --group lint --active --inexact

Or this can be done with pip:

uv pip compile --group lint > requirements-lint.txt
pip install -r requirements-lint.txt
bash format.sh

Now you are good to go 🚀

[tjohnson31415]

I took the liberty of updating most of the warmup logging: using a [WARMUP] tag in the message, more consistent formating, and combining adjacent log messages. But let me know if the preference would be to limit the changes to just update the "additional prefill" CB step.

[prashantgupta24]

LGTM! Couple nits

[yannicks1]

lgtm, if possible address @prashantgupta24 nits..

[joerunde]

get in!