M7: Bumblebee conformance breadth — ViT + Whisper - #14

Ports Bumblebee.Vision.VitTest verbatim — three tiny-random
architectures (:base, :for_image_classification,
:for_masked_image_modeling), same synthetic pixel input, same
expected slices. First conformance suite to exercise the conv
fallback path in anger.

Surfaced a latent bug in Emily.Backend.via_binary/via_binary_tuple:
the helpers transferred input tensors to BinaryBackend but left the
global default backend as-is. When a fallback op (Nx.conv in this
case) constructs a scalar tensor internally — e.g. the zero-pad
scalar via `Nx.pad(t, 0, ...)` — that scalar landed on
Emily.Backend whenever the conformance setup_all had swapped the
global default. The resulting mixed-backend operand list crashed
inside BinaryBackend's op with a FunctionClauseError on
to_binary/1 (the Emily-backed scalar's `data` doesn't match the
`%{state: data}` pattern BinaryBackend uses for its own tensors).

Fix: pin the default backend to Nx.BinaryBackend for the duration
of the fun call in both via_binary and via_binary_tuple. Never
surfaced before because test/emily/backend_fallbacks_test.exs
doesn't install Emily as the global default — it constructs
operands with an explicit `backend: Emily.Backend` — and every
earlier Bumblebee suite (DistilBERT, Qwen3) had already moved its
hot-path ops off the fallback by M4.

Full-size ViT-Base forward pass on Emily.Backend, pinned against
a deterministic synthetic pixel input. Structure mirrors
qwen3_full_test.exs: @moduletag :vit_full, excluded even from
--only conformance because the checkpoint is ~330 MB. Run with
`mix test --only vit_full`.

Uses a constant-gray pixel tensor rather than a checked-in JPEG
fixture so the repo stays free of binary test assets and the
featurizer preprocessing path doesn't enter the assertion surface —
the intent is to catch numerical drift on real-size weight tensors,
not verify image preprocessing. Pinned values: leading 5 logits +
argmax index 763 (ImageNet class "revolver") on the gray input.

Also update test/test_helper.exs to exclude :vit_full and
:whisper_full (landing next) from the default test run; expand the
comment to document each heavyweight tag and its cache footprint.

Ports Bumblebee.Audio.WhisperTest verbatim — two tiny-random
architectures (:base, :for_conditional_generation), same synthetic
mel features (Nx.sin(Nx.iota({1, 60, 80}))), same decoder inputs,
same expected slices.

First conformance suite to exercise encoder-decoder cross-attention
on Emily.Backend, and the first with strided 1-D conv in the
encoder frontend. Both pass first try against Bumblebee's
HF-Transformers-produced reference slices, which means the
cross-attention KV-cache update and the two stacked 1-D convs are
numerically consistent with PyTorch — despite conv still routing
through the BinaryBackend fallback (M8).

Full-size Whisper-tiny forward pass on Emily.Backend, pinned
against a deterministic synthetic 30 s mel window (sin(iota({1,
3000, 80}) * 0.01)) and a short special-token decoder prompt.
Asserts the leading 3×3 logits slice at 1e-3 tolerance plus the
decoder-last-step argmax (== 50257, <|endoftext|>, which is what
the synthetic input collapses to — that's fine for a pin; the
assertion is "same backend + same weights + same input reproduces
the same token", not "the model says something interesting").

Runtime is ~7 minutes on Apple Silicon because Emily.Backend.conv
still routes through Nx.BinaryBackend (deferred to M8). Whisper
encoder has two 1-D convs (kernel=3, out=384) over 3000 time-steps,
~940M multiply-adds executed as BEAM loops. The full test is
:whisper_full tagged, excluded from --only conformance, and opt-in
only; M8 wiring Backend.conv to Native.conv_general will bring this
to sub-second.

Also picks up formatter fixups on lib/emily/backend.ex (the
via_binary assignment from the previous commit) and trailing blank
lines on distilbert_test.exs / qwen3_test.exs.

- Renumber milestones: M7 = conformance breadth (this), M8 =
  native conv (lift Backend.conv off the BinaryBackend fallback),
  M9 = 1.0 release (was M7).
- M7 section documents the two new models and the rationale
  (vision, encoder-decoder, conv coverage) and records that
  MoE / Mixtral is deferred pending upstream Bumblebee support.
- RELEASE.md accumulates the added test suites, the shared
  ConformanceHelper, the test_helper exclude-list update, and
  the via_binary default-backend fix surfaced by ViT tiny-random.