[lang] Warn about non-contiguous gradient tensors (#8450)

Fixes: #8443

### Brief Summary

copilot:summary

### Walkthrough

copilot:walkthrough

---------

Co-authored-by: pre-commit-ci[bot] <66853113+pre-commit-ci[bot]@users.noreply.github.com>

python/taichi/lang/kernel_impl.py

@@ -763,6 +763,16 @@ def call_back():
                     if v.requires_grad and v.grad is None:
                         v.grad = torch.zeros_like(v)
 
+                    if v.requires_grad:
+                        if not isinstance(v.grad, torch.Tensor):
+                            raise ValueError(
+                                f"Expecting torch.Tensor for gradient tensor, but getting {v.grad.__class__.__name__} instead"
+                            )
+                        if not v.grad.is_contiguous():
+                            raise ValueError(
+                                "Non contiguous gradient tensors are not supported, please call tensor.grad.contiguous() before passing it into taichi kernel."
+                            )
+
                     tmp = v
                     if (str(v.device) != "cpu") and not (
                         str(v.device).startswith("cuda") and taichi_arch == _ti_core.Arch.cuda