Calculate base_score based on input labels for mae. (#8107)

Fit an intercept as base score for abs loss.

tests/cpp/helpers.h

@@ -451,5 +451,16 @@ class RMMAllocator;
 using RMMAllocatorPtr = std::unique_ptr<RMMAllocator, void(*)(RMMAllocator*)>;
 RMMAllocatorPtr SetUpRMMResourceForCppTests(int argc, char** argv);
 
+/*
+ * \brief Make learner model param
+ */
+inline LearnerModelParam MakeMP(bst_feature_t n_features, float base_score, uint32_t n_groups,
+                                int32_t device = Context::kCpuId) {
+  size_t shape[1]{1};
+  LearnerModelParam mparam(n_features, linalg::Tensor<float, 1>{{base_score}, shape, device},
+                           n_groups);
+  return mparam;
+}
+
 }  // namespace xgboost
 #endif