[FRONTEND] Rename tl.reduction -> tl.reduce and improve testing (#1521)

`tl.reduction` is currently tested indirectly through the existing
reduction operators, but it's good to have a direct test for the
function itself.

---------

Co-authored-by: Philippe Tillet <phil@openai.com>

docs/python-api/triton.language.rst

@@ -96,9 +96,13 @@ Reduction Ops
     :toctree: generated
     :nosignatures:
 
+    argmax
+    argmin
     max
     min
+    reduce
     sum
+    xor_sum
 
 
 Atomic Ops