What are the pros and cons of joining as a senior resident (Community Medicine) in All India Institute of Medical Sciences?

Which AIIMS are you referring to? There are several now!

It is always a BIG plus to work in AIIMS, New Delhi. Not only do you get to work closely with the doyens of the discipline, but also gain an invaluable toehold in the city, which is pretty much the hub of public health excellence in India. There are a few cons, specially the fact that if it is AIIMS, New Delhi, you will probably be sent away to two years to the rural outpost. The pro of that is that there are ample opportunities to do good research and keep your clinical skills sharpened by treating patients at the secondary healthcare level.
About the other AIIMS, I do not have much idea! A good idea would be to approach someone already working there to get an idea of how things are. Nowadays there are several channels on WhatsApp groups and Facebook which can be used to find such a person.

And finally, these discussions on pros and cons are moot until you specify what your goal is. Like in the exchange between Alice and the Cheshire Cat in Lewis Carroll’s immortal Alice in Wonderland book:

“Would you tell me, please, which way I ought to go from here?”
“That depends a good deal on where you want to get to,” said the Cat.
“I don’t much care where—” said Alice.
“Then it doesn’t matter which way you go,” said the Cat.
“—so long as I get SOMEWHERE,” Alice added as an explanation.
“Oh, you’re sure to do that,” said the Cat, “if you only walk long enough.”

All the best in finding your place in the world!