Add pipeline tag

This PR improves the model card by adding the `pipeline_tag: image-to-3d` to the metadata. This will help users discover the model when filtering by task on the Hugging Face Hub.

README.md

@@ -1,5 +1,6 @@
 ---
 license: cc-by-nc-sa-4.0
+pipeline_tag: image-to-3d
 ---
 
 # 4DGT Model Card