[cleanup][3/x] unify dynamic input and grad_output casting

[vkuzo]

Summary:

As titled, removes redundant logic for (input|grad_output) + dynamic scaling

Test Plan:

./test/float8/test_everything.sh

Reviewers:

Subscribers:

Tasks:

Tags:

[vkuzo]

Stack from ghstack (oldest at bottom):

• [cleanup][4/x] unify weight casting #1481
• -> [cleanup][3/x] unify dynamic input and grad_output casting #1480
• [cleanup][2/x] split float8 mm by delayed vs dynamic #1461
• [cleanup][1/x] make hp_tensor_to_float8_dynamic only work with hp inputs #1458

[pytorch-bot]

🔗 Helpful Links

🧪 See artifacts and rendered test results at hud.pytorch.org/pr/pytorch/ao/1480

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