Add block_info to csrc

[LoserCheems]

This pull request introduces a new BlockInfo struct in the csrc/src/block_info.h file to manage sequence length and padding information for query and key sequences. The struct is designed for use in GPU kernels, with support for variable-length sequences and efficient offset calculations.

Key Changes:

New BlockInfo Struct:

• Added a templated BlockInfo struct with a boolean template parameter Varlen to handle variable-length sequences. It includes logic for initializing sequence-related parameters like cumulative sequence lengths, actual sequence lengths, and padding. ([csrc/src/block_info.hR1-R49](https://github.com/flash-algo/flash-sparse-attention/pull/4/files#diff-394b0098afb828601be7aad41de4a4125220493eb2b3bf4e768797a39757beaeR1-R49))

Offset Calculation Methods:

• Introduced q_offset and k_offset methods to compute memory offsets for query and key sequences based on batch and row strides. These methods are optimized for GPU execution with __forceinline__ and __device__ qualifiers. ([csrc/src/block_info.hR1-R49](https://github.com/flash-algo/flash-sparse-attention/pull/4/files#diff-394b0098afb828601be7aad41de4a4125220493eb2b3bf4e768797a39757beaeR1-R49))

Namespace and Header Updates:

• Wrapped the new struct in the FLASH_NAMESPACE namespace for modularity and included necessary headers like namespace_config.h. ([csrc/src/block_info.hR1-R49](https://github.com/flash-algo/flash-sparse-attention/pull/4/files#diff-394b0098afb828601be7aad41de4a4125220493eb2b3bf4e768797a39757beaeR1-R49))