Polish reshape error message under @to_static

[Aurelius84]

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File "/workspace/env3.7/lib/python3.7/site-packages/paddle/fluid/layers/nn.py", line 6308, in get_attr_shape
    "please turn it into a Tensor under @to_static. See above example for details." % dim_idx)

AssertionError: Only one dimension value of 'shape' in reshape can be -1. But received shape[2] is also -1.

    # N = x.shape()[2]              # N is an int. (NOT recommend under @to_static)
    N = paddle.shape(x)[2]          # N is a Tensor. (Recommend)
    z = paddle.reshape([N, -1, 4])  # z.shape is [-1, -1, 4]

If your target shape in Reshape represents dynamic shape,please turn it into a Tensor under @to_static. See above example for details.

[paddle-bot-old]

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[JZ-LIANG]

LGTM