fix: give appropriate name to CONCRETE_INPUT

[deep-diver]

for the serving signature, CONCRETE_INPUT remained as pixel_values. This is due to the early stage of development of this project. However, pixel_values is no longer the name of the Input of the model, so it should be modified appropriately.

[sayakpaul]

@deep-diver can this be closed?

[deep-diver]

I am now checking this issue by giving an appropriate name