Revert "DatePicker: input display style changes to block, fixed #2018"

This reverts commit b3095a70.

examples/docs/en-US/date-picker.md

-<style>
-  .demo-date-picker .el-input {
-    width: 180px;
-    margin: auto;
-  }
-</style>
-
 <script>
   module.exports = {
     data() {

examples/docs/en-US/datetime-picker.md

-<style>
-  .demo-datetime-picker .el-input {
-    width: 180px;
-    margin: auto;
-  }
-</style>
-
 <script>
   module.exports = {
     data() {

examples/docs/en-US/time-picker.md

-<style>
-  .demo-time-picker .el-input {
-    width: 180px;
-    display: inline-block;
-  }
-</style>
-
 ## TimePicker
 
 Use Time Picker for time input.

examples/docs/zh-CN/date-picker.md

-<style>
-  .demo-date-picker .el-input {
-    width: 180px;
-    margin: auto;
-  }
-</style>
-
 <script>
   module.exports = {
     data() {

examples/docs/zh-CN/datetime-picker.md

-<style>
-  .demo-datetime-picker .el-input {
-    width: 180px;
-    margin: auto;
-  }
-</style>
-
 <script>
   module.exports = {
     data() {

examples/docs/zh-CN/time-picker.md

 <style>
-  .demo-time-picker .el-input {
-    width: 180px;
-    display: inline-block;
-  }
   .demo-box {
     .el-date-editor + .el-date-editor {
       margin-left: 10px;

packages/theme-default/src/date-picker/picker.css

@@ -3,6 +3,9 @@
 
 @component-namespace el {
   @b date-editor {
+    position: relative;
+    display: inline-block;
+
     .el-picker-panel {
       position: absolute;
       min-width: 180px;