Add support for explicit batch for all models

[ligaz]

Currently only the ONNX models support the explicit batch strategy. I have my own TRT model that has the NCHW input size and I need to be able to use it.

The fix is to remove the hardcoding of the ONNX models and check the input dimensions size. If the number of dimensions are 4 (NCHW) then we are in explicit batch mode.

Additionally the initialization of mModelType and mModelPath fields for Engine file types is done before loading the engine. Those fields are potentially used in the loading itself (they were used when we were checking the ONNX model type before the fix).

[ligaz]

@dusty-nv Can you do a review of these changes? Is there a better way to handle this scenario?

[dusty-nv]

Hi @ligaz, thank you for the PR - yes, I believe this looks good but have not had a chance to test it yet. I have added it to my TODO queue however to be merged/tested into the dev branch, and from there into master. Thanks again!

[ligaz]

@dusty-nv Do you want me to base this change on top of the dev branch?

[dusty-nv]

@ligaz it's not that big of a code change so I would probably just apply it to dev branch manually, but if you wanted to that is great too. What I need to do is test it against the non-ONNX models to make sure it doesn't inadvertently trigger any incorrect behavior with those