add python wrapper for sequence_pool

[luotao1]

fix #6777
The generated sequence_pool website is :

[luotao1]

The generated sequence_first_step website is :


The generated sequence_last_step website is :

[pkuyym]

Is it necessary to limit the type in (average/sum/sqrt/max)?
If so, I think it's better to add a check.

[luotao1]

If I limit the type in (average/sum/sqrt/max), then sequence_first_step should not be

def sequence_first_step(input, **kwargs):
    return sequence_pool(input=input, pool_type="first")

It should be

def sequence_first_step(input, **kwargs):
    helper = LayerHelper('sequence_pool', input=input, **kwargs)
    dtype = helper.input_dtype()
    pool_out = helper.create_tmp_variable(dtype)
    max_index = helper.create_tmp_variable(dtype)

    helper.append_op(
        type="sequence_pool",
        inputs={"X": input},
        outputs={"Out": pool_out,
                 "MaxIndex": max_index},
        attrs={"pooltype": "first"})

    return pool_out

but max_index = helper.create_tmp_variable(dtype) could not be removed. If we want to remove it, we should add an single sequence_max_op.cc

[pkuyym]

I see, thanks.

[pkuyym]

Great Job