Update pipeline tag

This PR ensures NVLM can be viewed from https://huggingface.co/models?pipeline_tag=image-text-to-text&sort=trending, along with friends like Pixtral, LLaVa, Qwen2-VL,...

Thanks!

README.md

@@ -2,7 +2,7 @@
 license: cc-by-nc-4.0
 language:
 - en
-pipeline_tag: text-generation
+pipeline_tag: image-text-to-text
 tags:
 - nvidia
 - NVLM