As a Web Designer wants to learn how to design for mobile devices do I need to learn ios/android Development ?

If its web pages, then I would say no.

But would recommend the CSS courses and learn about responsive design.

If you want to build native apps, then yes, I would go down the route of iOS and Android development

No, you don't need to learn iOS and/or Android development. But you may want to read the UX documents that are provided by Apple and Google.

iOS: https://developer.apple.com/library/ios/documentation/userexperience/conceptual/mobilehig/

Android: https://developer.android.com/design/index.html

In most mobile project teams that I have been involved in during the last six years, the designer(s) never had any first hand development experience. But of course, in a perfect world it would help a lot if the designer had an idea about how his design may complicate a developer's life. sometimes it's small design decisions that multiply the development effort exponentially.