[R-package] Prevent remembering parameters (#796)

* Enable simple cloning of dataset

* Add remodel logical to Dataset

* Allow remodeling of data

* Missing comma

* Use name instead of assignment for R6 class

* There should be no need to deep copy ALL objects

R-package/R/lgb.Dataset.R

 Dataset <- R6Class(
   classname = "lgb.Dataset",
-  cloneable = FALSE,
+  cloneable = TRUE,
   public = list(
     
+    # Logical to check whether a dataset can be used re-modeled in-memory as another Dataset or not
+    remodel = TRUE,
+    
     # Finalize will free up the handles
     finalize = function() {
       
@@ -276,6 +279,9 @@ Dataset <- R6Class(
         stop("lgb.Dataset.construct: label should be set")
       }
       
+      # Forcefully block construction
+      self$remodel <- FALSE
+      
       # Return self
       return(invisible(self))
       

R-package/R/lgb.train.R

@@ -153,7 +153,10 @@ lgb.train <- function(params = list(),
   }
   
   # Construct datasets, if needed
-  data$construct()
+  if (data$remodel == TRUE) {
+    data <- data$clone(deep = FALSE)
+    data$construct()
+  }
   vaild_contain_train <- FALSE
   train_data_name <- "train"
   reduced_valid_sets <- list()