Thanks for your answer. I have already seen this post. The solution provided here mentions that:
you would be training from the scratch.
Which beats the purpose of using pretrained model, If I am not wrong.  Does this approach make sure that model using the pretrained models weights ?
And the 2nd solution of making grayscale to rgb , I mentioned already. If the information is same why I would like to use 3x computational resource and time ?
One should be able to use only grayscale image.