fix: Improve input handling for input_signature

[gs-olive]

Description

• Improve error messaging for input signature arguments by verifying the nesting depth of the object is within the bounds specified in the TorchScript compiler
• Throw user-error more quickly to avoid less-interpretable error messages later on

Relevant GraphInputs code:

TensorRT/core/ir/GraphInputs.cpp

Lines 42 to 54 in 5fa6374

	if (level == 0) { // a single value like A
	collection_inputs.resize(1);
	collection_inputs[0].push_back(cur_input);
	} else if (level == 1) { // like A in [A, A] or [(B, B), A]
	collection_inputs[index].push_back(cur_input);
	} else if (level == 2) { // like A in [(A, A), C]
	collection_inputs[index].push_back(cur_input);
	} else { // only support 2 level
	LOG_ERROR(
	"Input nesting depth exceeds currently supported depth (3), use 1 level: [A, B], or 2 level: [A, (B, C)]");
	}
	}
	}

Fixes #1595

Type of change

• Improved input handling + error messaging

Checklist:

• [ x ] My code follows the style guidelines of this project (You can use the linters)
• [ x ] I have performed a self-review of my own code
• [ x ] I have commented my code, particularly in hard-to-understand areas and hacks
• [ x ] I have made corresponding changes to the documentation
• [ x ] I have added tests to verify my fix or my feature
• [ x ] New and existing unit tests pass locally with my changes
• [ x ] I have added the relevant labels to my PR in so that relevant reviewers are notified

[narendasan]

Is 2 arbitrary or is there some sort of technical depth limit? If possible i think it would be nice to support deeper nesting like 3-4 but not a massive deal. LGTM

[gs-olive]

Is 2 arbitrary or is there some sort of technical depth limit?

The choice of maximum depth of 2 is based on the data structure used to store the inputs: std::vector<std::vector<Input>>. While this structure could be expanded via adding more nested vector structures, a more robust solution would be desirable (supporting arbitrarily complex collections).