Custom logsumexp (#2028)

* initial custom logsumexp

* more tests

* comments + fix

mlx/backend/metal/jit/includes.h

@@ -20,6 +20,7 @@ const char* copy();
 const char* fft();
 const char* gather_axis();
 const char* hadamard();
+const char* logsumexp();
 const char* quantized();
 const char* ternary();
 const char* scan();