TransformerDecoder should support single inputs

[chenmoneygithub]

Currently TransformerDecoder must have both encoder_inputs and decoder_inputs, while for models like GPT2, only decoder_inputs is required. We should make a change to mark encoder_inputs as optional.

[jessechancy]

I'll like to take this issue

[chenmoneygithub]

@jessechancy Thanks!

[kanpuriyanawab]

Is this issue still looking for contributions ?
@chenmoneygithub @mattdangerw

[mattdangerw]

This is fixed!