[einsum] refactored and supporting unknown shapes in static mode

[tongxin]

PR types

New features

PR changes

APIs

Describe

Refactor einsum to support the static mode execution. Operands with unknown shapes are allowed.

Example,

paddle.enable_static()
a = paddle.static.data(name='a', shape=[3, None, None, None], dtype='float')
b = paddle.static.data(name='b', shape=[2, None, None, None], dtype='float')
c = paddle.einsum("ibnd,jbnd->bnij", a, b) 

[paddle-bot-old]

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[Aurelius84]

LGTM