Pakistan cannot build a nuclear sub because of the amount of research that would go into miniaturization of the nuclear reactor. Pakistan has not initiated reserach into it.
For always neutral: India did not get help on the miniaturization of the reactor for its ATV. They got help from Russia on the design of the sub.
Thirdly, with or without nuclear sub, PN cannot counter IN. Whilst you say Neo, that after the modernization and expansion of PN, it would be able to, you are forgetting , IN is modernizing (though it is already quite modern-relatively) as well and is expanding at a ferocious rate.
What you are unable to get is, in case of a war with PN, PN maybe able to take out some of IN ships, but considering the vast size of IN, it would sure as hell not make a dent. The IN would OTOH wipe out the PN, with only the subs remaining, and IN specialises in ASW as you very well know. They place prime emphasis on ASW.
IN is not IAF or IA, they are expanding and modernizing at a pace that cannot even be matched by the other two, which is sad because to modernize and expand a Navy, it take far more time than for AF or Army.
If you have read our Navy's new doctrine, their focus is on affecting the land and air battles through the sea. You have aircraft carriers , BrahMos AshM's and LACM's that would affect the land battle as well. Not to mention the fact that the Navy wants to put the BrahMos on the subs as well. And then comes the LPD's. There is a reason why IN is going the whole hog when it comes to this, wanting 3 Aircraft carriers, numerous LPD's and BrahMos , etc on subs. This is not to mention the DDG's that would be able to fire on land.