Check shape and remove deprecated APIs in scheduling_ddpm_flax.py (#7703)
`model_output.shape` may only have rank 1.
There are warnings related to use of random keys.
```
tests/schedulers/test_scheduler_flax.py: 13 warnings
/Users/phillypham/diffusers/src/diffusers/schedulers/scheduling_ddpm_flax.py:268: FutureWarning: normal accepts a single key, but was given a key array of shape (1, 2) != (). Use jax.vmap for batching. In a future JAX version, this will be an error.
noise = jax.random.normal(split_key, shape=model_output.shape, dtype=self.dtype)
tests/schedulers/test_scheduler_flax.py::FlaxDDPMSchedulerTest::test_betas
/Users/phillypham/virtualenv/diffusers/lib/python3.9/site-packages/jax/_src/random.py:731: FutureWarning: uniform accepts a single key, but was given a key array of shape (1,) != (). Use jax.vmap for batching. In a future JAX version, this will be an error.
u = uniform(key, shape, dtype, lo, hi) # type: ignore[arg-type]
```