Improve Instructions for pooling_layer

[GrierPhillips]

• [x ] Are you running the latest bert-as-service?
• [x ] Did you follow the installation and the usage instructions in README.md?
• [x ] Did you check the FAQ list in README.md?
• [x ] Did you perform a cursory search on existing issues?

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Description

The current documentation is slightly confusing regarding the use of the pooling_layer argument. For example, the documentation states:

pooling_layer	list	[-2]	the encoding layer that pooling operates on, where -1means the last layer, -2 means the second-to-last, [-1, -2] means concatenating the result of last two layers, etc.

However, upon some testing the only correct way to launch the server with concatenation is as follows:

bert-serving-start ... -pooling_layer -4 -3 -2 -1

The documentation would lead one to believe this is passed as a list, but in fact it is an int with nargs='+'. Perhaps consider updating the documentation to make this more clear?

[cpmss521]

@jacobdevlin-google ,

[ayxemma]

why it doesn't work when passed in from python?
args = get_args_parser().parse_args(['-pooling_layer', "-2 -1",...])

[acadTags]

I had the same question as the above.

To concatenate the hidden states of last 4 layers, the right way in python for -pooling_layer is

args = get_args_parser().parse_args(['-model_dir', model_path,
                                         '-pooling_strategy', 'NONE',
                                         '-pooling_layer', '-4','-3','-2','-1',
                                         '-max_seq_len', 'None',                                         
                                         '-cpu'])

Thanks,
A