Add missing conv2d validation steps

[zolkis]

          @zolkis For options, how about adding the validation steps for the `else` condition?  

If options.strides is undefined, set it to [1, 1].

If options.[strides] is undefined, set it to [1, 1]. else if options.strides.size() != 2 or any element in strides is equal to 0, throw error.
For detailed algorithm, please refer to the implementation in chromium code- https://source.chromium.org/chromium/chromium/src/+/main:third_party/blink/renderer/modules/ml/webnn/ml_graph_builder.cc;l=202.

Originally posted by @lisa0314 in #407 (comment)