Wrapper class for ESPnet module

[Masao-Someki]

As the ESPnet supports more and more models, we want to support them as quickly as possible.
To achieve this fast development, we are planning to create a wrapper class for the ESPnet modules.
This class will replace some modules to the onnx version.
(We need to replace some modules because we cannot simply export them to onnx format.)

I'm planning to use the wrapper class as:

from espnet_onnx.export import ESPnet2OnnxWrapper

enc_wrapper = ESPnet2OnnxWrapper(espnet_model.encoder, **config)
enc_wrapper.export(
    output_path='path/to/the/model',
    quantize=True,
    optimize=False,
)

and in the ESPnet2OnnxWrapper class,

from espnet_onnx.export import 

class ESPnet2OnnxWrapper:
    def __init__(self, *args):
        # register replacement map here or in other file
        replace_map = {
            espnet2.asr.encoder.TransformerEncoder': OnnxTransformerEncoder
        }
        self._replace() # replace TransformerEncoder into OnnxTransformerEncoder

In this way, what the developers do to support new model is:

1. Create onnx class if the module is unable to export.
2. Register the class into the dictionary