Fix complex VJPs for log and exp

[CameronChurchwell]

Fix complex VJPs for log and exp

This is the same as #3433, just with the exp and log functions now. Specifically, the VJP implementations for these primitives were lacking a conjugation which is necessary for correctness under the Wirtinger convention used by MLX. The change to Log::vjp looks more extensive since the previous implementation re-uses Log::jvp, but I have effectively copied over and modified the body of Log::jvp, so the actual change is just the addition of the conjugation

I have added tests to verify correctness, and I ran pre-commit already. No documentation changes should be necessary, as this is just a correctness fix.

Checklist

Put an x in the boxes that apply.

• I have read the CONTRIBUTING document
• I have run pre-commit run --all-files to format my code / installed pre-commit prior to committing changes
• I have added tests that prove my fix is effective or that my feature works
• I have updated the necessary documentation (if needed)

[zcbenz]

Looks good to me, thanks!