Inconsistency between `read` and `write`

[weitsang]

It seems that read takes in the name of the directory (which is required). No --input-dir is necessary.

For write, however, we need to specify --output-dir. This means that the name of the output directory is optional.

[YoHoSo]

Currently, --output-dir does not have a default value, so it is not optional.

[weitsang]

In this case, maybe we can remove the option --output-dir so that users specify the output directly, just like read

[YoHoSo]

it is consistent now, updated in dfcd3d4