Created by: zhiqiu

PR types

Bug fixes

PR changes

APIs

Describe

paddle.stack does not support a single Tensor as input anymore.

Before this PR, paddle.stack accepts a single Tensor x as input, and implicitly convert it to a list of Tensor [x].

This is not a good design since it violates the single responsibility. The responsibility of stack is to stack several Tensors, so this PR prohibits that usage.

An Exception, input as Variable of LOD_Tensor_Array type in static graph is still ok, since it means array of tensors.

For example, the following code will result in an error.

import paddle

paddle.disable_static()
x1 = paddle.rand([2,2])
result = paddle.stack(x1, axis=0)

• dygraph

Traceback (most recent call last):
  File "test_stack.py", line 8, in <module>
    result = paddle.stack(x1, axis=0)
  File "/usr/local/lib/python3.5/dist-packages/paddle/tensor/manipulation.py", line 468, in stack
    return layers.stack(x, axis, name)
  File "/usr/local/lib/python3.5/dist-packages/paddle/fluid/layers/nn.py", line 10055, in stack
    return core.ops.stack(x, 'axis', axis)
paddle.fluid.core_avx.EnforceNotMet: 

--------------------------------------
C++ Traceback (most recent call last):
--------------------------------------
0   paddle::platform::EnforceNotMet::EnforceNotMet(paddle::platform::ErrorSummary const&, char const*, int)
1   paddle::platform::GetCurrentTraceBackString[abi:cxx11]()

----------------------
Error Message Summary:
----------------------
InvalidArgumentError: stack(): argument 'X' (position 0) must be list of Tensors, but got Tensor (at /Paddle/Paddle/paddle/fluid/pybind/op_function.h:88)

• static graph

Traceback (most recent call last):
  File "test_stack.py", line 8, in <module>
    result = paddle.stack(x1, axis=0)
  File "/usr/local/lib/python3.5/dist-packages/paddle/tensor/manipulation.py", line 468, in stack
    return layers.stack(x, axis, name)
  File "/usr/local/lib/python3.5/dist-packages/paddle/fluid/layers/nn.py", line 10058, in stack
    raise TypeError("The type of '%s' in %s must be %s, but received %s" % ( 'x', 'stack', 'list[Tensor] or tuple[Tensor]', type(x)))
TypeError: The type of 'x' in stack must be list[Tensor] or tuple[Tensor], but received <class 'paddle.fluid.framework.Variable'>