Update book refs to release/0.21 branch

Change pwff activation to `#[module(skip)]` for backward compat (stat…

…eless) (#4929)

Update version to 0.21.0-pre.5 (#4921)

* Update version to 0.21.0-pre.5

* Update tests for n_fft assert

Update version to 0.21.0-pre.4 (#4890)

Fix burn-dispatch and burn-vision publish

Enable ndarray in burn-dispatch by default

Add missing burn-rl metadata (#4496)

Bump versions (do not pin exact version match)

Fix book guide training changes (#4340)

Feature gate tracing