[SPARK-15172][ML] Explicitly tell user initial coefficients is ignored when size mismatch happened in LogisticRegression

## What changes were proposed in this pull request?
Explicitly tell user initial coefficients is ignored if its size doesn't match expected size in LogisticRegression

## How was this patch tested?
local build

Author: dding3 <dingding@dingding-ubuntu.sh.intel.com>

Closes #12948 from dding3/master.

(cherry picked from commit a78fbfa)
Signed-off-by: Xiangrui Meng <meng@databricks.com>

Author: dding3

mllib/src/main/scala/org/apache/spark/ml/classification/LogisticRegression.scala

@@ -367,9 +367,10 @@ class LogisticRegression @Since("1.2.0") (
           Vectors.zeros(if ($(fitIntercept)) numFeatures + 1 else numFeatures)
 
         if (optInitialModel.isDefined && optInitialModel.get.coefficients.size != numFeatures) {
-          val vec = optInitialModel.get.coefficients
+          val vecSize = optInitialModel.get.coefficients.size
           logWarning(
-            s"Initial coefficients provided $vec did not match the expected size $numFeatures")
+            s"Initial coefficients will be ignored!! As its size $vecSize did not match the " +
+            s"expected size $numFeatures")
         }
 
         if (optInitialModel.isDefined && optInitialModel.get.coefficients.size == numFeatures) {