Update per_sample_grads.py

Fix for : https://github.com/pytorch/tutorials/actions/runs/16319435104/job/46093166038?pr=3463#step:9:5868

Author: sekyondaMeta

intermediate_source/per_sample_grads.py

@@ -79,6 +79,7 @@ def loss_fn(predictions, targets):
 #   pass to get an individual (per-sample) gradient.
 
 def compute_grad(sample, target):
+    model.zero_grad()
     sample = sample.unsqueeze(0)  # prepend batch dimension for processing
     target = target.unsqueeze(0)