Add support for method level executorch backend config

[tarun292]

Summary: There are use cases where we might like to supply a separate ExecutorchBackendConfig for each method in the model. An example use case is where we might want to alloc inputs for one method and not alloc them for another. In order to support this, in this diff we add support for passing in a dictionary of configs to to_executorch.

Reviewed By: cccclai

Differential Revision: D56499598

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As of commit d6c1a3d5e65fb8559f44169089eae68580e30c2a with merge base ebdb152aba547f17ef2952d105658e16afb70a0b (): :green_heart: Looks good so far! There are no failures yet. :green_heart:

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This pull request was exported from Phabricator. Differential Revision: D56499598

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This pull request was exported from Phabricator. Differential Revision: D56499598

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This pull request was exported from Phabricator. Differential Revision: D56499598

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This pull request was exported from Phabricator. Differential Revision: D56499598

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This pull request was exported from Phabricator. Differential Revision: D56499598

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This pull request was exported from Phabricator. Differential Revision: D56499598

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This pull request was exported from Phabricator. Differential Revision: D56499598

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This pull request was exported from Phabricator. Differential Revision: D56499598

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This pull request has been merged in pytorch/executorch@2ac7f2a84ddb3ad573af5488a03f5afa6d342318.