fix(pt): Fix PairTabAtomicModel OOM error (#3484)

Reduce memory usage of `_extract_spline_coefficient`.

Signed-off-by: Jinzhe Zeng <jinzhe.zeng@rutgers.edu>

deepmd/pt/model/atomic_model/pairtab_atomic_model.py

@@ -415,20 +415,28 @@ def _extract_spline_coefficient(
         # (nframes, nloc, nnei)
         expanded_i_type = i_type.unsqueeze(-1).expand(-1, -1, j_type.shape[-1])
 
-        # (nframes, nloc, nnei, nspline, 4)
-        expanded_tab_data = tab_data[expanded_i_type, j_type]
-
-        # (nframes, nloc, nnei, 1, 4)
-        expanded_idx = idx.unsqueeze(-1).unsqueeze(-1).expand(-1, -1, -1, -1, 4)
-
         # handle the case where idx is beyond the number of splines
-        clipped_indices = torch.clamp(expanded_idx, 0, nspline - 1).to(torch.int64)
-
+        clipped_indices = torch.clamp(idx, 0, nspline - 1).to(torch.int64)
+
+        nframes = i_type.shape[0]
+        nloc = i_type.shape[1]
+        nnei = j_type.shape[2]
+        ntypes = tab_data.shape[0]
+        # tab_data_idx: (nframes, nloc, nnei)
+        tab_data_idx = (
+            expanded_i_type * ntypes * nspline + j_type * nspline + clipped_indices
+        )
+        # tab_data: (ntype, ntype, nspline, 4)
+        tab_data = tab_data.view(ntypes * ntypes * nspline, 4)
+        # tab_data_idx: (nframes * nloc * nnei, 4)
+        tab_data_idx = tab_data_idx.view(nframes * nloc * nnei, 1).expand(-1, 4)
         # (nframes, nloc, nnei, 4)
-        final_coef = torch.gather(expanded_tab_data, 3, clipped_indices).squeeze()
+        final_coef = torch.gather(tab_data, 0, tab_data_idx).view(
+            nframes, nloc, nnei, 4
+        )
 
         # when the spline idx is beyond the table, all spline coefficients are set to `0`, and the resulting ener corresponding to the idx is also `0`.
-        final_coef[expanded_idx.squeeze() > nspline] = 0
+        final_coef[idx > nspline] = 0
         return final_coef
 
     @staticmethod