Upgrade coremltools, transformers; remove attn workaround

[pcuenca]

Transformers 4.30.0 replaced the _build_causal_attention_mask method with a function (thus harder to patch) that could not be converted to Core ML because of apple/coremltools#1915. The PR was released as part of coremltools 7.0b2, and therefore we can:

• Upgrade coremltools.
• Unpin transformers, which had been frozen to 4.29.2. We now require 4.30.0 or better.
• Remove the custom _build_causal_attention_mask in the conversion code.

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[atiorh]

Thanks @pcuenca!