Mock optimum, torchao, gguf for diffusers docs (#806)

With DIFFUSERS_SLOW_IMPORT=yes (which the shared workflows set), the
quantization docs need the quanto/torchao/gguf backends importable.
Mocking optimum also sidesteps a diffusers bug in the unavailable path:
dummy_optimum_quanto_objects uses backend name "optimum_quanto" while
BACKENDS_MAPPING only has "quanto", so requires_backends raises
KeyError instead of the intended ImportError.

Co-authored-by: Claude Fable 5 <noreply@anthropic.com>

Author: mishig25

src/doc_builder/mock_deps/diffusers.txt

@@ -12,3 +12,6 @@ real:pillow
 real:transformers
 bitsandbytes
 real:sentencepiece  # kolors tokenizer imports it at module level
+optimum  # optimum-quanto backend for quantization docs (also avoids diffusers' optimum_quanto/quanto BACKENDS_MAPPING KeyError in the dummy path)
+torchao
+gguf