Add test debug features (#4144)

Summary:
- Functions to get unbuilt default CompileSpec to change it before passing to ArmTester
- dump_operator_distribution to print a list with all operators in the graph and the number of times they appear.
- dump_dtype_distribution to print a list with the dtype of all placeholders and the number of times they appear.
- Cast data in tensor to float and correct shape if tensor dtype is FP32 when dumping Partition artifact.

    Signed-off-by: Erik Lundell <erik.lundell@arm.com>
    Change-Id: I7196527d060ba182b8ada8e48535d4bb7681ab68

Change-Id: I4678d19a40d5ee6ccab68798fdec7090db0eb8f8

Pull Request resolved: https://github.com/pytorch/executorch/pull/4144

Reviewed By: cccclai

Differential Revision: D59568414

Pulled By: digantdesai

fbshipit-source-id: 1c5928b6c6d1969ad497e0dd29e2d28fcb441cde