Poplar can't lookahead in the application runtime because the user drives the data pipeline.

Summary:
What's changed:
- Limit the lookahead of the application runtime.
  - This is because the data input is driven more directly by the user's python code.

Ref T50668

Test Plan: CI tests

Reviewers: #tensorflow, #framework_ip_review_-_any_oss_or_third-party_code_use_has_been_approved, jorgec, georgep

Reviewed By: #tensorflow, #framework_ip_review_-_any_oss_or_third-party_code_use_has_been_approved, jorgec, georgep

Subscribers: georgep, jorgec

Maniphest Tasks: T50668

Differential Revision: https://phabricator.sourcevertex.net/D57128

Author: jakeh-gc

tensorflow/compiler/plugin/poplar/kernels/application_runtime/application_runtime.cc

@@ -646,7 +646,10 @@ class EngineResource {
                  PoplarExecutableProto& proto)
       : engine_name_(engine_name),
         device_(std::move(device)),
-        engine_(std::move(executable)),
+        // Setting streamCallbacks.maxLookahead because we can't control the
+        // user data inputs in the application runtime. This means we can't
+        // gaurantee to poplar it can lookahead arbitrarily.
+        engine_(std::move(executable), {{"streamCallbacks.maxLookahead", "0"}}),
         communication_manager_(proto, timeout_us) {}
 
   poplar::Engine& GetEngine() { return engine_; }