How can I create a negation of the sentence? What are some ways that I can generate the negation of a sentence such that the output sentence reflects the negation of original sentence?

If the sentence is: That is an apple.

Then, the expected negation could be: That is not an apple.

Another example:

Sentence: Rita is so hot.
Negation: Rita is boring.
You could look in to Style Transfer. This is an active field of research.