Flow Matching Euler Scheduler added

[leffff]

In PR I have implemented a scheduler for Flow Matching with Euler method.

The popularity for Flow Matching models grows rapidly. Even novel models like SD3 (https://arxiv.org/pdf/2403.03206) use flow matching instead of Diffusion algorithm.
The FlowMatchingEulerScheduler features a full range of methods. However, unlike regular diffusion, in Flow Matching, timesteps are continous, but not discrete, therefore x_t = t * x_1 + (1 - t) * x_0. To infer Flow-Mathcing models one should perform Euler Method x_t = h * f(x_{t-1}, t) + x_{t-1}.

My implementation and usage examples could be seen here: https://github.com/leffff/euler-scheduler