Enable saturateHost in image-classifier and address a couple resulting bugs (#3443)

Summary:
This PR enables saturate host in image-classifier, so now if multiple devices are created they will be used.
As a result of this change bugs in OpenCL and CPUFunctions were exposed this PR fixes those too.

Documentation:
Pull Request resolved: #3443

Test Plan: run image-classifier with num-devices >0 and backends= CPU, Interpreter, and OpenCL.

Differential Revision: D16941867

Pulled By: gcatron

fbshipit-source-id: 8fe1fb4d777c8dc5fa1b8d88e14fbee01dc0ed89

Author: gcatron

include/glow/LLVMIRCodeGen/LLVMCompiledFunction.h

@@ -53,6 +53,10 @@ class LLVMCompiledFunction : public CompiledFunction {
   /// The LLVM JIT engine. The jit must be initialized after the ctor
   /// initializes the LLVM backends.
   std::unique_ptr<llvm::orc::GlowJIT> JIT_;
+
+  /// The JIT can be accessed from multiple threads but is not thread safe,
+  /// JITLock_ protects it.
+  std::mutex JITLock_;
 };
 } // end namespace glow