Update IREE tf-nightly version to the latest one

To match https://github.com/openxla/iree/pull/14725

Change-Id: I504b1451a1a52b1a63c2aaffc5525e7b86d8ff05

python-requirements.txt

diff --git a/python-requirements.txt b/python-requirements.txt
index d15022f..5a3b6e7 100644
--- a/python-requirements.txt
+++ b/python-requirements.txt
@@ -35,6 +35,6 @@
 sel4-deps
 setuptools
 # TODO(lundong): not pin nightly after TOSA features are in release ver.
-tf-nightly==2.14.0.dev20230705
+tf-nightly==2.15.0.dev20230817
 wget
 yapf