[SYCL] This PR adds the minimum modifications to the Eigen unsupported module required to run it on devices supporting SYCL.

* Abstracting the pointer type so that both SYCL memory and pointer can be captured.
* Converting SYCL virtual pointer to SYCL device memory in Eigen evaluator class.
* Binding SYCL placeholder accessor to command group handler by using bind method in Eigen evaluator node.
* Adding SYCL macro for controlling loop unrolling.
* Modifying the TensorDeviceSycl.h and SYCL executor method to adopt the above changes.

unsupported/Eigen/CXX11/src/Tensor/TensorDeviceThreadPool.h

@@ -75,6 +75,11 @@ struct ThreadPoolDevice {
   EIGEN_STRONG_INLINE void deallocate_temp(void* buffer) const {
     deallocate(buffer);
   }
+  
+  template<typename Type>
+  EIGEN_DEVICE_FUNC EIGEN_STRONG_INLINE Type get(Type data) const { 
+    return data;
+  }
 
   EIGEN_STRONG_INLINE void memcpy(void* dst, const void* src, size_t n) const {
 #ifdef __ANDROID__