If you google "define variant" and "define variation", you get:
A form or version of something that differs in some respect from other forms of the same thing or from a standard.
A change or slight difference in condition, amount, or level, typically within certain limits
A different or distinct form or version of something
The definition of variant is basically the same as the second definition of variation, so wherever you see the word "variant", you could replace it with the word "variation" and it would still have the same meaning.
All the examples you gave use the second definition of variation, so it would mean exactly the same thing to say
- I see it as another human variant
- Another variant of this problem is the use of validators
- The quagga was a strikingly beautiful variation of the zebra.
- Note that you must use the same variation of this option for all compiler runs related to the same project.
However, the first definition of variation is different to the definition of variant, so you could say
There is a lot of variation in the prices of stocks
but you could not say
There is a lot of variant in the prices of stocks.