Add sentencepiece and protobuf for the T5 tokenizer

requirements.txt

@@ -11,6 +11,10 @@ peft>=0.13
 safetensors
 huggingface-hub
 qwen-vl-utils
+# SD3's third text encoder is T5. Its fast tokenizer is built from a sentencepiece model, and
+# the conversion wants protobuf. Both arrive transitively in the research env, not here.
+sentencepiece
+protobuf
 lpips
 einops
 pyyaml