Context free grammar for L={ ((ab)^n)^m }
No need for a CFG: this language is regular!

$$L = (ab)^*$$

Any number of $ab$s is in the language: just set $n$ to 1, and $m$ to the number of repetitions. So all you need is a Kleene star.

If you really want a CFG, you can do it like this:

$$S \rightarrow SX \mid \varepsilon$$
$$X \rightarrow ab$$

This is the "standard" way of translating a Kleene star into a CFG.