Fix taking higher-order derivatives of cond_v2.

The problem:
When we build the N-th derivative of an op or set of ops, we will
likely end up reconstructing the previous (N-1)-th derivatives (we
could theoretically avoid this by cleverly finding and reusing
previously-constructed gradients as we traverse the forward pass).

In the case of the If op, this means that we end up constructing the
same gradient functions multiple times when taking higher-order
derivatives. Prior to this change, we would always generate the same
function name for the same grad function.

This usually worked because the two functions would be identical, and
we already silently dedup identical functions (this is to ease
importing graphs with functions). However, it occasionally didn't work
because we ended up generating two different FunctionDefs with the
same name (I'm not sure why the FunctionDefs were different, but I'm
guessing it's the unordered_map in the TF_GraphToFunction
implementation).

The solution:
Rather than depend on the subtle deduping behavior, I made the cond_v2
implementation find unique names for all grad functions. This will
result in more functions being generated, but I think it makes the
behavior more obvious.

In addition, this change properly adds the If branch functions to the graph.

PiperOrigin-RevId: 199560887