Meta implementation for nms (#7944)

Signed-off-by: Edward Z. Yang <ezyang@meta.com>
Co-authored-by: Philip Meier <github.pmeier@posteo.de>

torchvision/_meta_registrations.py

 import functools
 
 import torch
+import torch._custom_ops
 import torch.library
 
 # Ensure that torch.ops.torchvision is visible
@@ -48,3 +49,17 @@ def meta_roi_align_backward(
         ),
     )
     return grad.new_empty((batch_size, channels, height, width))
+
+
+@torch._custom_ops.impl_abstract("torchvision::nms")
+def meta_nms(dets, scores, iou_threshold):
+    torch._check(dets.dim() == 2, lambda: f"boxes should be a 2d tensor, got {dets.dim()}D")
+    torch._check(dets.size(1) == 4, lambda: f"boxes should have 4 elements in dimension 1, got {dets.size(1)}")
+    torch._check(scores.dim() == 1, lambda: f"scores should be a 1d tensor, got {scores.dim()}")
+    torch._check(
+        dets.size(0) == scores.size(0),
+        lambda: f"boxes and scores should have same number of elements in dimension 0, got {dets.size(0)} and {scores.size(0)}",
+    )
+    ctx = torch._custom_ops.get_ctx()
+    num_to_keep = ctx.create_unbacked_symint()
+    return dets.new_empty(num_to_keep, dtype=torch.long)