Add FP8 placeholder support to ExecuTorch serialization #3364

Job	Run time
test-qnn / test-backend-linux (qnn, operators) / linux-job	14m 25s
test-qnn / test-backend-linux (qnn, models) / linux-job	26m 50s
test-qnn / test-backend-macos	0s
test-qnn / package-golden-artifacts	2m 21s
	43m 36s

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