setup_env.sh's group discovery used grep -E '^[a-z_-]+ = \[', which
excludes digits and made torch_2_8/2_9/2_10/2_11 invisible to
validate_groups — breaking test-highest-pytorch/test-lowest-pytorch
with 'Unknown dependency group', caught by the fork CI run.

setup_env.sh now reads TORCH_GROUP directly and folds it into every
sync it runs, so make env/test/test-fast/docs/etc. all pin torch
instead of floating on whatever pyproject.toml's plain bound resolves
to. test-highest-pytorch/test-lowest-pytorch/env-highest-torch lock
their group via an inline `export TORCH_GROUP=... &&` at the top of
the recipe (rather than a hardcoded --with-torch_2_X flag), so their
name stays a promise regardless of a command-line override — verified
that GNU Make's own `override` directive achieves the same lock but
trips up the mbake Makefile formatter's duplicate-target detector,
which doesn't recognize that syntax.

--with-torch_2_X stays as a guarded alternate path for direct,
non-Make invocations of setup_env.sh; a guard now errors clearly if it
disagrees with an explicitly-set TORCH_GROUP instead of surfacing a
raw uv conflicting-groups resolver error.

…ORCH_GROUP

Now that the Makefile exports TORCH_GROUP unconditionally, the
smoke-test-specific SMOKE_TEST_TORCH_GROUP env var was redundant
indirection. ci/nox/noxfile.py reads TORCH_GROUP directly, and
`make test-smoke` no longer needs to translate the Makefile variable
into a differently-named env var for the nox subprocess.

The custom bash check that detected TORCH_GROUP disagreeing with an
explicit --with-torch_2_X flag duplicated logic uv already provides:
`uv sync --group torch_2_11 --group torch_2_8` fails on its own with
a clear "Groups ... are incompatible with the conflicts: ..." error
and non-zero exit. Simplifies the --all-groups exclusion loop and the
plain --group append accordingly, removing the GROUP_REQUESTED/
TORCH_GROUP_ALREADY_SYNCED bookkeeping that only existed to feed it.

The Makefile always exports TORCH_GROUP before invoking nox, so the
Python-side default was dead code for every make test-smoke run;
kept only for a bare, non-Make nox invocation which isn't a supported
entrypoint.

The expanded rationale duplicated what's now self-evident from
reading the three lock-target recipes directly.

`export TORCH_GROUP=torch_2_X && ...` set the variable for the whole
recipe's shell, which happened to be harmless since nothing else in
these three recipes reads TORCH_GROUP, but the intent was always to
scope the override to just the one setup_env.sh invocation that needs
it. Using `TORCH_GROUP=torch_2_X <command>` as an inline prefix does
that precisely: it applies only to the first command in the `&&`
chain, reverting to the Makefile's own default/command-line value for
everything after.