Expose candle gather op in pyo3. (#1870)

candle-pyo3/py_src/candle/__init__.pyi

@@ -324,6 +324,12 @@ class Tensor:
         """
         pass
 
+    def gather(self, index, dim):
+        """
+        Gathers values along an axis specified by dim.
+        """
+        pass
+
     def get(self, index: int) -> Tensor:
         """
         Gets the value at the specified index.