Add --expt-relaxed-constexpr to surppress Eigen warnings

cmake/flags.cmake

@@ -115,7 +115,8 @@ set(COMMON_FLAGS
     -Wno-error=literal-suffix
     -Wno-error=sign-compare
     -Wno-error=unused-local-typedefs
-    -Wno-error=parentheses-equality # Warnings in Pybind11
+    -Wno-error=parentheses-equality # Warnings in pybind11
+    --expt-relaxed-constexpr # surppress warnings in Eigen
 )
 
 set(GPU_COMMON_FLAGS
@@ -130,6 +131,7 @@ set(GPU_COMMON_FLAGS
     -Wno-error=unused-local-typedefs
     -Wno-error=unused-function  # Warnings in Numpy Header.
     -Wno-error=array-bounds # Warnings in Eigen::array
+    --expt-relaxed-constexpr # surppress warnings in Eigen
 )
 
 if (APPLE)