Update en docs of to_tensor (#29718)

* update to_tensor en docs

python/paddle/vision/transforms/functional.py

@@ -62,11 +62,11 @@ def to_tensor(pic, data_format='CHW'):
 
     Args:
         pic (PIL.Image|np.ndarray): Image to be converted to tensor.
-        data_format (str, optional): Data format of input img, should be 'HWC' or 
+        data_format (str, optional): Data format of output tensor, should be 'HWC' or 
             'CHW'. Default: 'CHW'.
 
     Returns:
-        Tensor: Converted image. Data format is same as input img.
+        Tensor: Converted image. Data type is same as input img.
 
     Examples:
         .. code-block:: python

python/paddle/vision/transforms/functional_cv2.py

@@ -40,7 +40,7 @@ def to_tensor(pic, data_format='CHW'):
 
     Args:
         pic (np.ndarray): Image to be converted to tensor.
-        data_format (str, optional): Data format of img, should be 'HWC' or 
+        data_format (str, optional): Data format of output tensor, should be 'HWC' or 
             'CHW'. Default: 'CHW'.
 
     Returns:

python/paddle/vision/transforms/functional_pil.py

@@ -49,7 +49,7 @@ def to_tensor(pic, data_format='CHW'):
 
     Args:
         pic (PIL.Image): Image to be converted to tensor.
-        data_format (str, optional): Data format of img, should be 'HWC' or 
+        data_format (str, optional): Data format of output tensor, should be 'HWC' or 
             'CHW'. Default: 'CHW'.
 
     Returns:

python/paddle/vision/transforms/transforms.py

@@ -296,15 +296,21 @@ class BaseTransform(object):
 class ToTensor(BaseTransform):
     """Convert a ``PIL.Image`` or ``numpy.ndarray`` to ``paddle.Tensor``.
 
-    Converts a PIL.Image or numpy.ndarray (H x W x C) in the range
-    [0, 255] to a paddle.Tensor of shape (C x H x W) in the range [0.0, 1.0]
-    if the PIL Image belongs to one of the modes (L, LA, P, I, F, RGB, YCbCr, RGBA, CMYK, 1)
-    or if the numpy.ndarray has dtype = np.uint8
+    Converts a PIL.Image or numpy.ndarray (H x W x C) to a paddle.Tensor of shape (C x H x W).
+
+    If input is a grayscale image (H x W), it will be converted to a image of shape (H x W x 1). 
+    And the shape of output tensor will be (1 x H x W).
+
+    If you want to keep the shape of output tensor as (H x W x C), you can set data_format = ``HWC`` .
+
+    Converts a PIL.Image or numpy.ndarray in the range [0, 255] to a paddle.Tensor in the 
+    range [0.0, 1.0] if the PIL Image belongs to one of the modes (L, LA, P, I, F, RGB, YCbCr, 
+    RGBA, CMYK, 1) or if the numpy.ndarray has dtype = np.uint8. 
 
     In the other cases, tensors are returned without scaling.
 
     Args:
-        data_format (str, optional): Data format of input img, should be 'HWC' or 
+        data_format (str, optional): Data format of output tensor, should be 'HWC' or 
             'CHW'. Default: 'CHW'.
         keys (list[str]|tuple[str], optional): Same as ``BaseTransform``. Default: None.