[Executorch][Export][2/N] Add getter utils to help with getting the artifacts easily

[abhinaykukkadapu]

Stack from ghstack (oldest at bottom):

• [Executorch][Export][5/N] Remove portable lib dependency in et/export #16130
• [Executorch][Export][4/N] Add edge program transform stage which runs after to_edge() stage #16129
• [Executorch][Export][3/N] Add timing information to all the stages for debugging #16128
• -> [Executorch][Export][2/N] Add getter utils to help with getting the artifacts easily #16127
• [Executorch][Export][1/n] Add exported program and GraphModule as inputs, validation and tests #16126

Add getter utility methods to ExportSession for easier artifact retrieval.

• get_exported_program - to get output from export stage
• get_edge_program_manager - multiple stages produce edge program manager, for example, to_edge, transform, to_backend, go in reverse order to get the most processed edge program manager.

Differential Revision: D87576720

[pytorch-bot]

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❌ 1 New Failure

As of commit 867a66b with merge base 9cd8402 ():

NEW FAILURE - The following job has failed:

• pull / android / run-emulator (gh)
  Timeout waiting for emulator to boot.

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