Improve ChatML template

[BadisG]

Hello,

The ChatML template must look like this:

<|im_start|>system
{system_message}<|im_end|>
<|im_start|>user
{prompt}<|im_end|>
<|im_start|>assistant

But the current ChatML template gives this result

<|im_start|>system
{system_message}
<|im_end|>
<|im_start|>user
{prompt}
<|im_end|>
<|im_start|>assistant

This can lead to really incoherent outputs when talking to the bot.
I've also tweaked the code a bit so that when there's no system prompt, this disappears:

<|im_start|>system
<|im_end|>

[oobabooga]

That seems correct, thanks