Add gather api combined implementation (#730)

* Add gather api combined implementation

* Update gather api combined implementation

docs/pytorch_project_convertor/API_docs/ops/torch.gather.md

@@ -46,8 +46,8 @@ if axis == 1:
     将tensor_list中的tensor沿axis轴拼接
 ```
 
-
 ### 代码示例
+
 ``` python
 # PyTorch示例：
 t = torch.tensor([[1, 2], [3, 4]])
@@ -66,3 +66,34 @@ paddle.gather(t, paddle.to_tensor([1, 0]), 1)
 #        [[2, 1],
 #         [4, 3]])
 ```
+
+### 组合实现
+
+```python
+def paddle_gather(x, dim, index):
+    index_shape = index.shape
+    index_flatten = index.flatten()
+    if dim < 0:
+        dim = len(x.shape) + dim
+    nd_index = []
+    for k in range(len(x.shape)):
+        if k == dim:
+            nd_index.append(index_flatten)
+        else:
+            reshape_shape = [1] * len(x.shape)
+            reshape_shape[k] = x.shape[k]
+            x_arange = paddle.arange(x.shape[k], dtype=index.dtype)
+            x_arange = x_arange.reshape(reshape_shape)
+            dim_index = paddle.expand(x_arange, index_shape).flatten()
+            nd_index.append(dim_index)
+    ind2 = paddle.transpose(paddle.stack(nd_index), [1, 0]).astype("int64")
+    paddle_out = paddle.gather_nd(x, ind2).reshape(index_shape)
+    return paddle_out
+
+t = paddle.to_tensor([[1, 2], [3, 4]])
+paddle_gather(t, 1, paddle.to_tensor([[0, 0], [1, 0]]))
+# 输出
+# Tensor(shape=[2, 2], dtype=int32, place=CPUPlace, stop_gradient=True,
+#        [[1, 1],
+#         [4, 3]])
+```