ci: Use tox-current-env to reuse prepared venv with torch

[booxter]

This is a completion of a workaround for flash-attn. The initial attempt didn't work because we still used tox to run smoke tests, which means another venv was created that did not include flash-attn.

To achieve sharing the venv for both torch installation and smoke tests, we are using tox-current-env plugin here.

[RobotSail]

@booxter Is the problem that this fixes only in CI? Or would the end user be expected to run tox in the same way (i.e. having a venv)?

[booxter]

@RobotSail until Dao-AILab/flash-attention#958 is merged (and they don't seem keen to do so for reasons that I don't agree with), every project that depends on flash-attn has to pre-install their build dependencies, particularly pytorch. tox doesn't allow to order installation of dependencies to guarantee that torch is always installed before flash-attn, so I think any tox user is potentially affected by this. tox being a development tool and not something an end-user is supposed to use in production is probably a mitigating factor, but yes - this sucks. :(

[booxter]

@Mergifyio rebase

[mergify]

rebase

✅ Branch has been successfully rebased

[booxter]

Testing the change here: https://github.com/instructlab/training/actions/runs/14713551885

[booxter]

(The more I patch this, the more I don't like it. But let's see how horrible the final "fix" would look like...)

[booxter]

This run confirms that tox-current-env works now: https://github.com/instructlab/training/actions/runs/14719818289/job/41311408922

Now to decide if we want to see this solution. (If not, I'm all ears to hear about alternatives that would allow us to run smoke tests with [cuda] depenendencies that include the flash-attn library with broken build instructions.)