Hello @Mohsin_Saleem
With what the NLP tester mentioned you precisely towards right direction, I also want to add another point, to make sure you are doing the updated version of the assignment as I can see your grader cell looks totally different than mine.
I am sharing screenshot of my grader cell for Exercise 1 - line_to_tensor 
To get a fresh copy of the updated assignment, follow below steps
- Click file, then select open. You will find all the files related to the assignment. Go ahead and delete the assignment notebook by selecting the particular notebook.
- Once deleted, you will find 404 not found image on your browser. Then close the browser.
- Open the assignment page again, when you open you will find 404 not found.
- At this time, click Help on the right hand top corner, where you will find Reboot. Click reboot.
- Then click the same help on the right hand top corner, then click Get latest version and then Update lab.
Voila you have a fresh copy of your assignment notebook
Use below two images hints to resolve your code issue which are part of instructions given in the assignment
Regards
DP