am: add allocation bench (#10135)

* init allocation bench

* sorryg

* betetr

test/external/external_becnhmark_am.py

@@ -0,0 +1,9 @@
+from tinygrad.helpers import Profiling
+from tinygrad import Device
+
+if __name__ == "__main__":
+  am = Device["AMD"]
+
+  # kfd is 0.55ms!
+  with Profiling("allocation 127.7mb"):
+    am.allocator.alloc(int(127.7*1024*1024))