feat: expose sin_cache / cos_cache accessors on nn::RoPE

Add public `sin_cache()` and `cos_cache()` accessors that return the
pre-computed rotary embedding tables. Downstream code that wants to
apply RoPE without going through `RoPE::forward` (for example, a
dispatch that routes the computation to a different operator library)
needs these to reproduce the cached scaling — particularly for
LongRoPE, whose per-dimension short/long factors are non-trivial to
recompute outside this class.

No change to behavior; the new accessors expose the existing buffers
read-only.

Author: voltjia

include/infinicore/nn/rope.hpp

@@ -120,6 +120,11 @@ class RoPE : public Module {
     Algo algo() const { return algo_; }
     DataType dtype() const { return dtype_; }
 
+    // Accessors for the pre-computed sin/cos cache tables.
+    // Shape: `[max_seq_len, head_dim / 2]`, dtype: `dtype()`.
+    Tensor sin_cache() const { return sin_cache_; }
+    Tensor cos_cache() const { return cos_cache_; }
+
     // String representation
     std::string extra_repr() const;