Support padding direction selection for collate (#5263)

add an optional boolean argument pad_rght to Pad class to support padding direction selection

PaddleNLP/paddlenlp/data/collate.py

@@ -81,6 +81,9 @@ class Pad(object):
             data type of returned length. Default: False.
         dtype (numpy.dtype, optional): The value type of the output. If it is
             set to None, the input data type is used. Default: None.
+        pad_right (bool, optional): Boolean argument indicating whether the 
+            padding direction is right-side. If True, it indicates we pad to the right side, 
+            while False indicates we pad to the left side. Default: True.
     Example:
         .. code-block:: python
             from paddle.incubate.hapi.text.data_utils import Pad
@@ -96,11 +99,12 @@ class Pad(object):
             '''
      """
 
-    def __init__(self, pad_val=0, axis=0, ret_length=None, dtype=None):
+    def __init__(self, pad_val=0, axis=0, ret_length=None, dtype=None, pad_right=True):
         self._pad_val = pad_val
         self._axis = axis
         self._ret_length = ret_length
         self._dtype = dtype
+        self._pad_right = pad_right
 
     def __call__(self, data):
         """
@@ -132,7 +136,11 @@ class Pad(object):
                 ret[i] = arr
             else:
                 slices = [slice(None) for _ in range(arr.ndim)]
-                slices[self._axis] = slice(0, arr.shape[self._axis])
+                if self._pad_right:
+                    slices[self._axis] = slice(0, arr.shape[self._axis])
+                else:
+                    slices[self._axis] = slice(max_size - arr.shape[self._axis], max_size)
+
                 if slices[self._axis].start != slices[self._axis].stop:
                     slices = [slice(i, i + 1)] + slices
                     ret[tuple(slices)] = arr