Falcon support

[KexinFeng]

The falcon models tiiuae/falcon-40b tiiuae/falcon-7b require special handling of the kv_cache, done in FalconBlock.

# concatenate along seq_length dimension:
# [batch, seq, num_heads*kvDim = 73 * 64]
#  - key: [batch_size * self.num_kv, seq, kvDim]. [2, 6, 64]
#  - value: [batch_size * self.num_kv, seq, kvDim]. [2, 6, 64]

# Falcon
# fused_qkv: [batch, seq, (num_heads=71 + num_kv=1 + num_kv=1) * kvDim]
# query_layer: [batch*num_heads=71,  seq, kvDim]
# key_layer  : [batch*num_kv, seq, kvDim].  num_kv=1
# value_layer: [batch*num_kv, seq, kvDim].  num_kv=1
# hidden_dim = 4544

Note:
The tokenizer of falcon models have bug. It cannot tokenize two strings with different lengths.