* Correcting some grammatical mistakes in the design docs

* Changing image to figure as per code review feedback

* Change pronouns as per code review feedback

* Correcting few more typos

* It is duplicated with `dim_`. We can use `dim_` to calculate `numel`
  everytime. It does not cost too much.
* `numel` is not initialized by constructor. Also, `numel` is hard to
  synchronize with `dim_`.

So just remove it.

* remove alias

* Also use `const DeviceContext&` all the time, to prevent `const_cast`

Fix #4169
Fix #3468
Fix #3475