Change `NHWC`/`NCHW` to `NWC`/`NCW` for conv1d

While working on 13105 I noticed that in the current code
base `conv1d` uses `NHWC`/`NCHW` which should really
be `NWC`/`NCW`.

This fix addresses this issue and keep `NHWC`/`NCHW`
compatible internally so that users will not be impacted.

Signed-off-by: Yong Tang <yong.tang.github@outlook.com>