SeamlessM4T: `test_retain_grad_hidden_states_attentions` is flaky

[gante]

See the related PR which added the is_flaky decorator: #28035

cc @ylacombe, to explore in case you have spare bandwidth :)

[amyeroberts]

@ylacombe Re-opening as this isn't resolved by #28060 (it just makes sure our CI doesn't break)

[ylacombe]

Alright, copying the reasons for flakiness from #28060 for trackability then!

After investigating the reasons for the test_retain_grad_hidden_states_attentions flaky failure, I realized the speech encoder attentions can be None with a non-zero probability when training=True.